Community Tip - You can change your system assigned username to something more personal in your community settings. X
Try exporting all the parts as a STEP file and use the option to make it a single part:
During the "save-as" "step" dialog, go to "File Structure" and select "Single File"
Then you can open the STP file as a part (PRT) and it will be one level. Insert that into the board assembly.