Confused about 35 mm adapters
 

If I had a 35mm adapter such as the Letus extreme on my EX1 would I be able to use my standard Nikon lenses from my still camera?
What advantages could I expect?

Yes ...
 

advantages really have to do with dof but also range depending on you lens.

Ok
 

Ok - I can understand the DOF but can I say use normal Nikob DX lenses and say 400mm tele lenses to extend the range of the EX1?

I don't think you'll get the 1.5 bump in reach, and I'm also not sure how the dx lens would behave. It may work fine as you're just projecting an image onto ground glass. I don't know, just speculation.

It will not act as a telextender. The 35mm lens throws an image on a ground glass roughly equivalent to angle of view of your lens on the 35mm camera. All the video camera does then is record that image from the ground glass. With some cameras you will be able zoom a bit more on the image to have a narrower telephoto effect, but it will likely be at the expense of resolution and grain.