Sure, if you build triangular-shaped battens that you glue partly to the existing sloped ceiling and partly with some blobs on the floor, it'll probably hold without any trouble. Then place a batten on the floor between each triangle, it doesn't need to be glued but will act as a base to screw the new plasterboard into. The boards will stabilize everything.
If you're lucky, you might find the studs that the current plasterboard is attached to. Try searching for the screws with a magnet or stud finder.