Put a video fisheye onto a DSLR?

I'm planning on buying a DSLR and am wondering if i could put a video fisheye on to the fliter of a 18-55mm lens( standard nikon lens)
http://opteka.com/optekax-treme58mm03xhdsuperfisheyelensforvideocamcorders.aspx

You need to check whether it is compatible. It may work on manual but may not focus automatically.

I presume you have checked the screw threads are identical.

If the thread on the fisheye converter is the same as that on the front of the 18-55mm, then yes. But the results might not be very good. Usually these things have cheap optics and create a huge amount of distortion and vignetting. I certainly don't think it's worth the price shown on the website.

If you have an adapter/stepping ring…

The Nikon lens is a 52mm thread, so you either need a fisheye adapter of the proper size, or you need a 52-58mm stepping ring.

Yes, it will (mostly) work.