I suspect this is related to the information found on slide 27 of the xPCB Release Differences document located at:
DXF when imported is placed within a cell. In order to select graphics within a cell you must enable "Allow Cell Graphics Edits" within Editor Control. Once the graphics within the cell can be selected, you can use the standard process of Ctrl-Double click to make a copy of this draw object within the cell, then change it to a Board Outline using the Draw Properties dialog.
Product Marketing Director
just what i was looking for