Sorry… but it would be helpful to have this feature also for single shell surfaces, no matter if they are based on geometry or not. If this sometimes leads to inaccurate results, is this also the case for planar surfaces? If not, maybe an additional/limited feature for extruding only planar shell surfaces could be useful.
The attached model is a nice example… First you create the cad model on how you plan the simulation (creating all the mid-surfaces which is a lot of work for such big models). Then you need to prepare the model in prepomax, finally you run the analyses… and it fails for convergence issues… 
Yes, actually i wish to have this feature to test the linked model with solid elements for the flanges. It would be so comfortable to be able to extrude them at this stage…
Idea:
Kind regards