if you UVW map before importing into zbrush, you may run out of sub-d levels too early. as each sub-d level has a uvw map. if you sculpt without uv's then none of the sub-d layers have uv's, which allows for more memory per object to be used. generally speaking i don't UV anything until after the retopo stage. it's not…
No, you can import it (or GoZ it). You can UV unwrap it in ZBrush, or Maya, or other program (before you cast normal, AO, etc. maps). You may end up retopologizing it anyway. You have to be careful when exporting from ZBrush because if you don't have the correct settings ZBrush will not export the UVs.