[Gmsh] Create prismatic layer around imported *.stl file

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Hello everybody,

I am new to this list and hope it is okay to start with a question.
I have already imported a *.stl file using the Merge option and am now struggling to create a prismatic layer around it. As far as I now, it can be done with the 
Extrude{Surface{surface_number};Layers{{number_of_layers},{height_of_the_layer}};}
command. Therefore, I first have to define a surface on my stl-geometry in order to refer to it. When I try to assign a elementary ruled surface, nothing happens (I can’t even highlight a surface). When I try to assign a physical surface, gmsh seems to accept it. At least the surface appears in the *.geo file. When I want to extrude the newly created surface, gmsh complains about an unknown surface 5.
My whole *.geo file looks like the following:

Merge „mygeometry.stl";

Physical Surface(5) = {2};

Extrude{Surface{5};Layers{{10},{0.5}};};

I would really appreciate any advice. Thanks a lot in advance,

Kate