Baseboard heat wiring
Under NEC 2014, you can leave the conductors alone and add (retrofit) a ground wire. 

It is not necessary for the ground wire to follow the same route as the conductors. However the ground wire must go to the ground bus in the same panel the heater is powered out of.

Circuits which come out of that same panel can share ground wires.  So you don't have to home-run all the way back to the panel, if  you can reach the ground wire of another circuit also served out of that same panel.  However the entire route of the ground wire must be as thick as the conductors, i.e. at least 12 AWG in this case.

If your jurisdiction hasn't adopted NEC 2014 yet, well... I don't know what to tell you.  I would investigate whether they're likely to.