This is possible as long as your PBXes can look to DS0 18 for the D-channel.
The only configuration change you may have to do in the TSU 120s is to make sure Robbed Bit Signaling is turned off on the DSX-1 port (Config > Port Config > 0.2 DSX-1 > ROB BIT SIGNL: Off)
If it is already off then there are no changes needed in the TSU 120s.
Thanks,
Patrick