Is it possible to get outlets from STL files?

I just tried to read outlets from STL files, but I get no domains from the STL reader:

TreeReader3D<int>* treeReader =
createUniformTreeReader3D(stlFile, N, limitDepth);
vector<Box3D> treeDomains = (*treeReader).getDomains();
    cout << treeDomains.size();
// => 0

Before I go into full debugging: Is it possible to leave out the bounceback nodes behind an inlet or outlet, if there is only very little space between it and the next bounceback boundary - and it completely cuts the bounceback wall, so no fluid can flow around it?

That way I could simply place the in- and outlet sheets in blender and then import them as STL (which would make it much easier to adapt the simulation to other geometries).

Besides: My problem in my code is that I don’t get any domains from the STL file. Do I have to convert it to a boolMask or similar to see where there are nodes, or can I just make it solid? Or does it simply take a sheet as having no width and thus assigns no domain to it -> do I have to use a finite width “sheet” to enable STL reading?