Thanks a lot Lixus. Making doors is not that difficult after all!
I have on comment on this: The surface inspector dialog in Radiant looks a little bit different in newer versions. If the doorknob is on the wrong side, the entry in H Scale - Value has to be changed (plus or mins, just change the algebraic sign). Press "Unlock" first if the field is not editable (This is confusing if you do it the first time because the dialog in the Radiant manual is inaccurate).
Hopefully, this helps even newer newbies than I am to make decent doors
cheers
breli