Volume texture input options?

Is there a way to input anything other than a texture2d into a volume texture?
I’m trying to use a sprite sheet or flibbook to animate a volume texture but I can’t for the life of my figure out how do this. I saw that Ryan Brucks did some cool stuff with pseudo volume textures where he’s using render targets as inputs somehow. does anyone know how to set one of those up? That would be a good alternative. Thanks in advance.