Thank you Mike it works but it can not be a workflow it is workaround only.
Mike, Jacquelyn, consider this true life example. There is an empty dgn file (corridors container) where a few corridors 3D models are attached.
Architect who is cooperating with road design team has a read-only access to this file via ProjectWise. He just needs a single edge of sidewalk coppied into his own file. Not all lines drawn on specific level. Just one line form one sidewalk.
Following Mikes workflow, architect will merge all elements from choosen level. Then will need to delete all elements he doesn't need. Imaigne how many elements might be drawn on a single level in a complex corridor model.
I wish there was a kind of unlcock switch to copy elements between files.