Create a Manifold Solid from Blender Mesh Surface

SketchUp
2

I can export it as a dae or stl file and import it again in sketchup. So far no Problem.
When I try to a the skirt to create a volume, I can draw the lines, but it will not become an area.

Sketchup
Sketchup901×449 11 KB
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I tried the addons mentioned in this topic

https://forums.sketchup.com/t/make-mesh-surface-a-solid-use-stamp/106144/2

none of them work.

Any suggestions?