Phew...
I think I got it right this time.
After importing the original object, I sent it to the setup room, loaded the LF2DevRig and assigned the bones I needed.
Despite the initial advice, I chose auto grouping to have something to work with.
With those rough groups in place, I edited them nice and clean.
Back in the pose room, I renamed the piece.
The next step was to save the thing to my library and clear the viewport.
I then imported the object file with the recommended settings and did the same as I exported the file, overwriting the original object file.
Back in Blender, I imported the object file created by Poser and checked the weld and groups and BINGOOOO!

It worked!

Now I can die in peace. ;)

Just one additional question if I may.
I learned that the new garment works better if it inherits the joint zones from LF2DevRig instead of LaFemme2.
I checked this and it is indeed true.
My question would be whether it makes sense to add this step after rigging and before saving the part to a library.
Are these joint zones saved during this process?

Anyway, thanks for your help.

I will write this process down so I never have to ask this question again, I promise.
Greetings from Germany