Rather than convert RR to DB, or DB to RR, I'd think that changing both to critical reduction would work better. . .ala "Reduce all X crits by Y steps".
That would essentially work out with the logic of Warl's answer, which is that if an elf had "Reduce Cold crits by 2 steps" as their cold resistance, then they would take A Cold crits as A-50 cold crits, meaning they're unlikely to be harmful. . .but if you hit them with a cold bolt or ball, they would be just as easy to hit as a human of the same armor type or DB, and take the impact related hits the same way, but take a lesser cold critical. . .
Thus, a C critical caused by a cold wall, one caused by a frost blade, or one caused by a cold bolt, cone or ball would all be reduced in the same manner, in the same proportion, two steps to A criticals.
Of course, that would mean re-jiggering all the resistance racial abilities, and all the spells that offer resistance, which is a bit of a PITA. . .though you could just use a conversion table.