I have the same problem - having set the lathe up in inches (all the dials are calibrated in inches anyway)
You can just insert g21 before the section you require in metric (mm). This operates the machine in metric dimensions, but seems to use all the offsets etc that you have set up, in inches.
At the end of the section g20 reverts everything back to inches.
The toolpath seems a bit odd - because it just stays the same size. The DRO's revert to mm for metric, but revert to inches when you go back.
Seems to work alright.