How do I treat these BG tiles like sprites?

I guess one of the easiest way to do it is to apply GRP5 to sprite with SPPAGE 5.

Load "grp4:filename" Save the background Then use load command above with that files name and you be able to use them as normal sprites instead of background

Ah ok, so that means I can't use my sprite sheet and the BG sheet at the same time. Well, it's good to know though. :P

Under sprite edit function try copying from one sheet to the other