I doubt it would be practical to directly port Compiz (I don't know enough about WMs to give you definitive answer as to if it's possible). As to adding or modifying effects to Fremantle's WM, I'm not certain, but I wouldn't be surprised if it's possible. I frankly don't see much use, though - I think the N900 strikes a decent balance between flashy and usable, but too much eyecandy on a phone is not terribly useful. That said, if you or someone else wants to do it, go for it! I won't use it, but this is FOSS, and I don't have to agree.