(NOTE: TuneRegistry does not provide legal advice. It is your responsibility to ensure that a song is in fact a cover song as defined by copyright laws and that you've followed the appropriate steps to secure a cover song license.)
However, on the Song Details page, you must select the Yes button on the question "Is this a cover song?". The remaining details on the page will be hidden as you should not complete song ownership and publishing details for cover songs.
Then, add the recording in the Recordings section as you normally would. Because you owner the recording, you'll need to complete the Recording Details page.
You can register the recording, but you cannot submit registrations on the song.