Like Wade says' I just use any type of dry red I have for a red wine. No more than you add it won't affect the profile. I made a Vintners Reserve Cabernet Sauvignon about 3 years ago that was just too thin for me to enjoy. It made a great top off wine though. I have 2 bottles left.
You could use a Grenache, Syrah/Shiraz or Mourvedre, which are the primary grapes in a Chateauneuf du Pape (or Castel del Papa) or a blend of the same.
As dfw says, Castel del Papa is a Chateauneuf du Pape, a name copyrighted by the French. The kit makers can't use it, but they can duplicate the grape blend used to make it.
I agree with dfwwino, those choices of wine would be acceptable. If it were me, and I needed to top off with more than one bottle, I would get a bottle of shiraz and a grenache.