Certainly.
You'd have to configure the FF800 first, connected to your computer via Firewire, then routing its analog inputs to its ADAT outputs in TotalMix, and saving this configuration to its internal memory so that it is retained after you've disconnected it from the computer and made the ADAT connection to your UFX.
Due to the limitations of ADAT, you will only be able to use eight channels per ADAT connection (the FF800 has two available) at the lower sample rates - 44.1 or 48kHz. - or four channels each at 88.2 or 96, and two each at 192. Details are in the manual, which I believe can be obtained as a PDF download from RME if your used unit does not come with it.
Frank Lockwood
https://LockwoodARS.com
Fireface 800, Firmware 2.77
Drivers: Win10, 3.125; Mac, 3.36