In the GECK you can do that yes, but im fallout 3/NV the player and all npcs are the same in terms of model editing, like you can make anyone look like anyone else just with a small tweak within the GECK, in saints row (3 or 4 at least) the player and npcs are modeled differently itll be a case of converting someones head to become playable? Not to sure on that one