To answer your question, you can pick FX outputs in the “source select” area of the “preamp” tab on the “processing” screen.
There’s no need to use regular input channels for that, though. If you select the FX return channels and go to the “routing” screen, you can send them to monitor mixes the same as any other channel. Just be sure you have the FX return channel selected, not the FX send, or the routing screen will show you what’s being sent to that particular FX unit.