I have some Honda CVT transmission fluid left over from my previous car (2007 Honda Civic). My current car does not have a CVT, but an ATF transmission instead (2004 Honda Civic). I know its bad to use ATF fluid in a CVT transmission, but is it ok to use CVT fluid in an ATF transmission?
No, you should not under any circumstances be adding the incorrect fluid to a transmission. If the transmission in question calls for ATF use ATF. Likewise, if it calls for CVT fluid use CVT fluid. The couple dollars you might save by using some left over fluid will quickly be erased by transmission failure at worst, or needing a flush at best.