Under core/tables (if you've unpacked the bos files) you should find a file called campaign.txt right at the bottom is the list of prefabs. Add your prefab to the list following the same format. Then load up the BOS.cam file (you'll need to unpack it) in the campaign section of FT Tools. At the top click on tools and then choose import prefabs this will open up a list of documents, choose the campaign.txt you've edited. Save the cam file and now your prefab will have been added. You might want to back up the cam file first.