The terminology is somewhat confusing, and ther is nearly no documentation on doing this, but, it appears that you eed to add a slot from the command line on your cell manager
For example suppose your File Library is named "wadwho-02", and the media you want to add is in the directory " /datadomain/wadwho-02" and the filename is "970b02bf053942b08059f5001b3.fd". The command would be
omnimm -add_slot "wadwho-02" "/datadomain/wadwho-02/970b02bf053942b08059f5001b3.fd"
I would recommend, if the command runs without error, that you run
omnidownload -list_libraries -detail
to see if the slot made it into the library