The 3DS homebrew scene is still young, you'll have to wait for someone to port ScummVM to the 3DS.
It's sure to happen eventually, things like these always get ported to every console possible.
By the way - Anybody know if the 3DS+Ninjhax exposes enough RAM to be able to run the later LucasArts games with Scumm - possibly even Curse of Monkey Island?