Sigma, Tokina, Tamron, Cosina and Carl Zeiss have lens with Nikon mounts. Please check the sub-forum for Sigma, Tokina & Tamron and Nikon for more information
Yes you can, just make sure the lens is for the same mount for your cam and you'll be able to use it. Do note that some lenses may not be able to AF with your cam.
TS, please go and do some research on the lens prices from this forum and buy only from reputable shops. I am very afraid that your carrot head may be next on the chopping board.