@Skadalilama, kevphil: thanks! I'm glad that you could find it useful
@black104: The used cube is not native between the tools of zBrush, but you could find it in the default "zProjects" folder as "DefaultCube.ZPR". In order to obtain the simplest form I used the "Reconstruct Subdiv" a couple of time and after I deleted the "High subdiv".
The extrusion with mask is a new feature of the 4R2 release, it is beautifully illustrated on the video "DUPLICATE AND EXTRUDE": the last but one on
THIS THREAD... basically if you mask part of your mesh and use the MOVE pressing "CTRL" you make an extrusion.