One problem is that cassettes with more than one gear are pretty wide, and might not fit in a BMX frame. If you got one to fit, then I think it'd just be a matter of attaching the derraileur so it didn't rotate (easy enough), and adjusting it to actually work (no way I'd tackle that one.)

I think you'd be better off just making do with one gear. In my experience, to have gears on a bike work properly you need pretty high quality stuff and a lot of skill to keep it working right. Maybe you can find one of those single speed beach cruisers from walmart, etc, and ride that instead of the BMX bike for commuting. It'll go faster easier because it's got bigger wheels, and maybe you could put on a bigger chainring.