Is there any way to create a surface geometry from shell elements in LS-PrePost?
I guess it depends what you mean by surface geometry. If you are talking about CAD like an .iges file, then no, PrePost does not have this functionality. However, you can save a shell part out as a .stl file as long as it contains only tria elements. If you have some quad elements you can split them using the Element tool -> Element Edit -> Split/Merge and select on of the options to split the quad into tria elements and use the selection box to select all of the elements you want to split.
You can then save out a mesh in File -> Save as -> Save Geom As and select .stl from the file type. You can then use this .stl to create an .iges file but it's a time intense process and requires specific software.