Its possible to use the phones outputs, but then you loose them for connecting phones.
A more flexible solution - and to have line level outputs again supporting again balanced (or unbalanced) connections - would be to use the 2 ADAT output ports - which are there for such purposes - and
A) to connect one or two ADI-2 FS which give you additional 1-2 high power phones outputs.
B) Another option would be to connect an 8-port AD/DA converter like the ADI 8 DS MKIII which is a 19" device which you can nicely put into a 3U rack together with the UFX II. I propose to use RU more than needed to keep a little bit space for better air flow between the devices).
The latter at about the same price of two ADI-2 Pro would give you a little port reserve and also A/D conversion.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13