Vegetables. Crayfish eat just about everything you give them. I'd stick with vegetables like squash and peas in addition to the pellets you're feeding. Remove the pea skins and squash seeds so you don't get a mess.
Pretty much anything, really.
They are even more of omnivores then we are, as they'll eat pretty much any plants at all, as well as living, dead, and decaying flesh.
It'd probably love some chicken or fish, it'd definitely like peas and such.