I don’t know about Maya, but in Max 2009 it’s built into the app. It’s probably the same with new versions of Maya so check the docs.
With your system specs, you may need to delete the non facing side of your model, in ZBrush, before you take it into Maya for the conversion. The achiles heel of the mrproxy process, from my experiences in Max, is that you have to take the mesh into Max to do the conversion to mrproxy. Obviously this means that I often have to break the model up into chunks, as Max bombs on my system with objs over 1 mil polys.
I have not found any way to convert it externally, though I heard XSI has a tool for this.
I don’t know how Maya works with mrproxies, maybe a maya expert can fill you in.