Yes, send the digital out of the R44 into the 7xx (702 and 722 anyway, not sure about the others). On the 722, using the menu, select analog input on the XLRs (I forget the exact menu options, maybe analog, digital, auto-select). The 722 will record from the XLR inputs, but will sync to the clock from the digital input (the R44). It should show a "D" or "L" or something on the 722, I forget exactly.
This will allow you to record 4 chanels on the R44 and an additional 2 channels on the 702/722, all synced to the same clock off of the R44. Note though, this will not do anything in terms of syncing up the start and stop times of the two decks, so you will still need to line up the 7xx recording with the R44 recording.