Yes you can !!! (kind of as expected but good to know anyway).
If you want to see a 30 second video of it happening, download the following from my Dropbox.
Track 1 - Main Mix taken from outputs 1+2 of Fantom-G into Inputs 3+4 of UR44
Track 2 - Bass taken from outputs 3+4 of Fantom-G into Inputs 5+6 of UR44
Track 3 - MP3 taken from stereo out of Samsung phone into Inputs 1+2 of UR44. Note: Channel 1 on my UR44 has a fault and you can see this on the left channel of the MP3 Audio track.