This is one of those times where it's really best to let them go. Yes, theoretically you could grind the residue down and reconstitute it. It's going to be extremely time-consuming, you're going to have to grind the dried paint very finely, it could end up costing as much or more than buying new paints, and the end result will be disappointing.
Instead, I suggest just buying new paints from a modern manufacturer. Not only does this save you time and frustration, modern paints really are far superior to the old RP stuff.