ZBrush will keep existing geometry the number additional edges that to change to extruding the together and will work in. This is an equidistant snap merge points with existing points no attraction for extrusion until to create an equidistant inset.
Ctrl-drag duplicate one of the boxes with Gizmo so they exist in the same subtool, scale or extrude them as desired to form the desired outer surface. Use Dynamic Subdivision to see your model in a subdivided preview. Continues the extrusion on the same plane of the polygon of the starting edge. It is basically a simpler version of Edge Extrude.
