You can easily do it if you use registrations. Let's assume you're at a tempo of 100, and you want to program an ending on, say, Reg button # 6.. To slow it down on Re button # 5, just reduce the tempo to about 93, on save it on #5. Now it will make a very nice transition in tempo down to 92, and your ending will also be at that tempo, UNLESS you want to make the ending even slower, (say down to 88). Just stop the ending , and reduce tempo to 88, and save it on button # 6.
If you're not familiar with Registrations, and what they can do, please check out the Lessons area..... it's nothing short of critical for playing better. I find that you don't want to reduce the tempo by more than about 10% as a guideline. BTW, if you just want to reduce the tempo in that one spot in the song, and then want to take it back to 100, you can do that with the button following the slowdown button.