Gday, I have to very crappy single coil pickups from my cheap strat, is it possible to make them into a humbucker?
Technically yes, but you would have to open up the pups to put the magnet in the bottom. Too much work, and it's not worth it. Besides, crappy single coils DO NOT make a good humbucker. You would be better off buying a new guitar, or if your strat plays really nicely then buy a set of pups.

Also, although everyone says it - what amp have you got? A crap guitar through a really good amp will sound LOADS better than a fantastic guitar through a mediocre amp.
you can make them like a humbucker, but making them into a real one would be way too much work...
to make them like one, just make sure one is RWRP (most middle position single coils are RWRP), and wire them in series.