Yes,it can be done.I use Helmsman Spar urethane.Just make sure you have the riser dust free.A light coat to start and wait at least 12 hours to recoat-regardless what the can says.A diamondwood riser may only need 4 coats.Some woods may need more.
You may see a little transition were your new finish meets the old.Don't worry about that.If your last coat gets a spot of dusk,lightly remove that with 320 or 400 grit wet or dry sand paper.
Now,let the finish cure for about a week and rub it down with a good stock rubbing compound.This will blend the new finish to the old and any place you light sanded.
Birchwood Casey has a rubbing compound,Brownells has their own brand and Midway USA should have some.FFF compound is too fine,just regular stock rubbing compound for a matt finish.