Hi,
I fear you have chosen the wrong way from the start: WB will recognize the presence of volumes and thus mesh them with solid elements.
You still have the possibility to import it in Classical: there, you have full choice on which face / surface to use, you can add auxiliary geometry in order to create a bar where you need, and so on.
Though I haven't done a "mixed-element" analysis in WB, it should work like that:
- volumes are meshed with solid elements
- wireframe ("pure-lines") is meshed with beams (don't remember how you take into account the section...)
- surfaces are meshed with shells
You have to make sure that you select the mixed-geometry import options.
You also have to pay attention to the DOFs compatibility between different element types (beams with shells should work OK because they generally both have 6 DOFs/node, while shells with solids can become very tricky...).
I seem to remember that the Help section about Meshing in Simulation has a pretty good description of this matter.
Regards