I'm not sure what you mean. CK2toEU4 and CK3toEU4 both support sunset invasion and we've modded it extensively so poor natives start reformed, can build boats, have tributaries and so on. I suggest you grab ck2toeu4 and run a test conversion, manually activating sunset. Then see if the output is something you had in mind.So some day I'd like the possibility to have games with my Sunset Invasion mod converted to EU4, including changes to the Americas setup - just like vanilla CK2 does when the Sunset Invasion DLC is active (though somewhat tweaked as my lore is quite different).
It is not going to happen in the immediate future (still need to learn C++), but I have been considering how that could work, repository-wise. Could I merge some code into your main repository, that detects my mod and changes the output accordingly (and otherwise does nothing); or would I have to make my own branch of the repo, and direct people to use that one instead of the main one?
As for general contributions, of course we accept them, usually the procedure is to fork the main repo, make your own changes and make a pull request.