Do you want to make one of the background characters in Street Fighter playable or play say Phantasy Star as a NPC from one of the towns?
For the former then it will be a long winded affair wherein you create more animations, fiddle with hitboxes, do moves and more.
For the latter then (possibly ignoring any conversation portraits and battle animations) you could possibly pull it off with a tile editor, some copy-paste action and the same with the palette, all of which would involve no assembly at all. Also instead of a tile editor you might be able to pull it off by editing pointers.
On the other hand you might be back at the thing above and have to program the game to accept the new character and do all sorts of things.
For 3d games it gets a bit easier for some hacks like this as animations are done to models and are not whole sprite sheets needed to be created.