The following comes from Pat Stanford, coordinator of the LA VectorWorks User Group and creator of the PodCAD PodCast:
“We did trouble shoot one problem that you might want to point out to your group.
A user said she could not put doors or windows into a drawing after importing a DWG file. What is the problem?
It is a name space problem. The imported file created classes called Door and Window. Since you can’t have two “objects” in a VW file with the same name, you could not put Door or Window objects into the file.
The solution is to rename the classes to something else. Even better use the prefix option when you import the file so that you don’t end up with name conflicts. Better still is to not draw in a file with imported data, but use a design layer viewport to pull the data in from a file used for only DWG import.”