No. Audio locks the riff tempo so you can't do a gradual slow down or speed up. You can do a step tempo jump by changing the tempo of the next Riff in the song.
There are instant drummers in which the actually material slows down. I think the RAW stuff might have something. But if I recall correctly, it's a very electronic oriented beat that does it.
You can't change tempo within RiffWorks but you can tweak things around with the likes of Audacity after you've mixed down. This will let you change the tempo without affecting the pitch.