You need a different flash than the SB-400. The SB-400 cannot work at all if not on the camera hot shoe, or on a hot shoe cable connected to the camera hot shoe (because the SB-400 menu it uses is in the camera, but if remote, it has none).
Specifically, you need one that is spec'd to work with the commander.
You could turn off the commander, and put the SB-400 on the hot shoe, or on a hot shoe cord (old SC-17 or new SC-28, which are short, it will be beside the camera, but which is where fill light should be), and then use the remote Yongnuo flash in its S1 slave mode. This will be manual flash in both flashes, not TTL.
One exception (which I am not familiar with) is that the Yongnuo YN-622 is a radio trigger that mimics and replaces the commander, and its hot shoe claims to support the SB-400.