Modify > Extrude > Extrude Members to Shells prompts you with the following dialog.
It generates a series of shells by extruding selected members along a global direction. You may specify a distance list and an extrusion direction for the generation of shells. The extrusion direction must not be parallel to the selected members. You have the option to automatically merge nodes and elements after extrusion. You should check this option unless duplicate nodes are explicitly permitted. You also have the option to automatically delete selected members after the extrusion.
The input in the above dialog is applied to the members in the screen capture below...
members used for extruding
...to generate the shells in following screen capture by extruding.
shells generated