The easiest method would be to actually add China on the map.
Speaking as someone that's been doing that (or, well, I redid the non-vanilla province layout post-HF and also have done a bunch of compatibility stuff) as part of a mod, that would be
anything but easy (though, admittedly, the devs might be a lot better at map stuff and PDS presumably would have more resources to throw at the problem if they decided to do it).
First off, you're expanding the map in at least one direction. Are you also adding pixels to the south? If so, you're going to have to update half of the existing position entries since those are calculated from the bottom left corner of the map. You can do that with a script (if you can create that script), but already you're changing other stuff because you wanted China on the map, and it
won't be the last time you've got to change old stuff. Also, since you're expanding the map, are you aiming for just China, or are you adding other parts of the Far East that would be relevant for China (or other realms in the east), e.g. Korea, Japan, Southeast Asia, and Indonesia? I don't think there's a good map projection that'd give you only China, though I suppose you could have wastelands covering e.g. Burma (because apparently you don't care about what happened there; I can already see a bunch of "Paradox hates [country]" threads spring up), which naturally would lead to requests to fill in those in the future (or to expand the map further, if you e.g. stopped before Japan).
After deciding on the map size, you'll have to create a new height map, create a new river map, create a new terrain map, and create a new tree map (which has really weird proportions compared to the other map files (devs, why is it scaled 1:8.04188481675 (in one direction; I don't remember the other and it isn't on the wiki) instead of 1:8?) and thus easily will mess up pre-existing trees). I don't have experience with any of those (since someone else did that work), but since there's no other PDS game it can be directly imported from (since EU4 (and other games) has a different map projection and you thus can't just copy-paste it) it probably isn't trivial.
At this point (and just filling in the extended provinces.bmp with white to ensure that the game will launch), you've got an expanded terrain map with a wasteland (or, technically, an inaccessible sea) outside the boundaries of the current map. That's obviously not what you're after, so you need to add a good number (read: hundreds) of provinces. That means researching the area to find out a sensible layout. I suppose the devs might make use of EU4's layout/research done by the EU4 team to some extent, but that's going to result in rather few provinces compared to what the rest of the map has, so you'll have to do research to expand on that.
With an idea for a layout, you'll have to draw the provinces on the map, one by one, each in a different colour (which thankfully already exist; it would take even longer if you needed to make sure that there were no conflicts with previous colours), which likely will take several iterations since you'll want to check how it looks in the game. You'll also need to add localization for every province to avoid having e.g. "PROV2110" instead of "Chang'an".
Once you've done
that, you've broken your huge wasteland into several smaller wastelands. Now you'll need to start making the relevant provinces into playable counties, which means creating the de jure structure
from scratch, with seven baronies per county (because of Prosperity), creating a province history file for
every county to set up the initial culture/religion/development setup (which requires research and presumably will require defining a bunch of new cultures/religions at a basic level), adding CoAs for
every new title (which requires research and the involvement of people that actually can do graphical stuff (which I doubt all the PDS content designers can do; I've seen some temporary art in some steams)), adding positions for every new title, and probably a few more small bits I've forgotten, and that's
before you consider the fact that you also will need to add (ideally historical) rulers for everything, that you'll probably need a bunch of adjacencies (strait crossings, etc.), some new geographical regions, a bunch of tech history, and that you've got to make sure that it all is in decent shape for the three early start dates (CM, tOG, IC) and
every date between 1066.9.15 and 1337.1.1.
At this point, you've probably got a working map and decent history files (if you managed to find good rulers). If you think you're
done, you're mistaken. Pretty much everything having to do with offmap China will be broken (since offmap titles can't be on the map), so you're going to have to redo that in a fashion that works with China on the map (and some of it might not be possible to have with China on the map) and that works well with the player possibly being the target (since China being on the map but unplayable would be rather unsatisfactory), the Black Death's spread patterns (and origin) will need to be updated, the Silk Road will have to be expanded, the new cultures/religions will need some features, and you'll probably need/want to do a bunch of other things to make the new area fun to play in (particularly since Taoists have rather little at present).
Oh, and even after you do all of the above (hopefully without sacrificing a lot of performance, since that would be a deal-breaker for many), there will still be people complaining because they never head east of the Levant and they think that anything unrelated to crusading or (Christian) kings is beyond the scope of the game (despite the devs having said that the name isn't meant to be taken as "The only thing we're doing in the game"), or they think that whatever has been added is unreasonably inaccurate (since there doubtlessly will have been concessions to making things playable, enjoyable, and understandable), or they've got some other reason to complain (justifiably or not).
Sure, having China (and other parts of the Far East) on the map would be interesting (at least as far as I'm concerned; as mentioned above, it
won't be to everyone's liking), but adding it is
far more complicated than a bunch of other options that would handle the offmap title issue in IC (e.g. giving it to later Tang (or Later Jin, since they were about to take over) and having it in a state civil war).
