Adding Animation of characters into an existing game?
First you would need to be able to (reasonably) accurately recreate the character / clothing / background. The difficulty of this would depend on the art process - is it Daz3d? is it HoneySelect/Koikatsu? is it hand drawn art of unique characters? is it art of well known anime/game characters? Then you also need the skills to create the animations - do you have existing art-drawing experience (for 2d art), or 3D animation experience (for daz/blender?)
If you are just talking about some of the standard Daz games - For some games there is a lot of information about what DAZ assets and morphs were used (see some of the game specific fan-art threads in here:
https://f95zone.to/forums/artwork.112/). But for others there is no information available, you would have to discover it / work out how to do the re-creation yourself - and this could be difficult!
Assuming you overcame all those difficulties: if you had some nice juicy movies ready to do, then you need to substitute it into the game. For Renpy, this could be done by creating a patch file that overwrites the existing relevant Labels and adds the movie display in place of the previous static image display. Unfortunately you would need to constantly update (or at least review for breakage) this patch each time there is a new release. For other game engines, i don't know how hard it would be to patch them.
