There are some mods that introduce plutonium processing and MOX fuel cells, such as this and this. My guess is that they're doing that by introducing new fuel types. Is it possible to include those fuel so that the locomotive can also consume them?

If they are in the same fuel category as the existing uranium fuel cells then the locomotives should be able to use them.

I had take a look at those mods, and apparently they introduce new fuel categories (MOX for PlutoniumEnergy, apm_nuclear_mox for amatorphasma's). IIRC vanilla nuclear fuel have nuclear fuel category, so the locomotive can't use the plutonium fuels