Adding ground to an existing conduit
Pull out the wire that's in conduit, (look it over, but it's probably just fine) pull in that wire and a ground. 

It's darn near impossible to pull in another wire without pulling the whole bundle out and back in. It's fairly straightforward to pull the whole bundle out, add a wire and pull back in, unless the conduit is too small.

Code-wise, (250.118) properly installed metallic conduit (RMC, IMC or EMT) is a ground, whether or not you personally like it. It's not copper, but it's got a large cross-sectional area and the fittings are designed to maintain contact if properly installed.

I would not consider Romex an upgrade. I've seen too much rodent/Romex interaction to love it much, and with the possible exception of wire old enough to use cloth-based insulation as primary there's little benefit to "new wire" over "old wire."