can the rear wheel negative camber be adjusted or its a factory thing?

 

The rear camber and toe setting is fixed. It cannot be adjusted at all.

 

I've seen some maximas (2001 SE) with 0 camber, why?

 

A lot of ***** and a torch? That would be my only guess.

 

I think you would do it with a bigass press but it isn't en exact science. I read about it here though.
You could also cut a slot on each side of center of the beam; through the top, front and back of the beam. Bend the beam up on each end to close the gaps and weld it back together. I'd want to build a jig for this because you could get some undesirable results if not very careful.