We just went the other way - converting a flat roof to a vaulted ceiling. I am pretty sure it is fairly easy, although I don't know if you can do it yourself.

Basically, you would build a frame around the celing (9ft) and drop in the rafters/joists to hold the sheetrock. The span would probably determine what material you would use so it won't sag. Remember you would likely need to re-route electrical and a/c. Hang sheetrock / tape/bed and paint and you are all done.

Not a quick project, but I think fairly easy. Good luck.