http://collider.com/batman-the-killing-joke-mark-hamill-voicing-joker/
In effect, he already did it in an Arkham Knight flashback scene (and presumably in the Batgirl DLC for that game, "A Matter of Family").
They only show the shooting, and he only talks a little about what happened afterward on occasion, however. So an actual animated movie would fill the void well. DC animated adaptations of comic material are generally pretty good, though I think this would be the first time Hamill has voiced the Joker in a movie per se. (Or rather, one that wasn't part of the DCAU per se: Mask of the Phantasm was DC's first animated movie, with a theatrical release, and despite some problems fitting it into canon afterward is part of the DCAU 'series' of TV shows and films.)
Still, he's been pretty busy voicing the Joker for three out of the five Arkham games, most recently while reprising his role as Luke Skywalker in the new Star Wars movie. (And whoever did the Joker's voice for the Arkham Origins games, gave an excellent protracted impression of Hamill's Joker, worthy of taking on the role afterward; same guy did it for the Injustice: Gods Among Us game(s), too.) Those can be, and have been, trimmed down to (epic) movie-length, and can be found on YouTube (and other video sites like Dailymotion).