Some of the BGG Firefly: the Game enthusiasts have looked into combining it with Firefly Adventures: Brigands & Browncoats, a cooperative skirmish game which you could in theory use to expand what the boardgame handles as a series of Misbehave cards. But (a) FFA isn’t really flexible enough without a lot of work designing scenarios, converting characters, and so on, and (b) it would make for a very long game.