They were filmed on proper film but the editing and effects shots are video only. So it is technically possible to put them on BluRay and maybe even widescreen, major issue is going to be the fact every episode will need to be completely re-edited and all the effects shots redone, from the Enterprise flyby to Picards earl grey materialising. It's the same for TNG, DS9, and Voyager. Only Enterprise was intended to be in widescreen HD.
There was a story from a couple of years ago of the people who did TOS remastering trying it out on a TNG episode to see what the quality and costs would be -
http://trekmovie.com/2007/07/27/cbs-testing-remastering-tng/ not heard anything since.
Personally I'd love to get a remastered HD version of DS9, maybe one day.