Not sure if "loopback" is the correct word...
Loopback is being able to use the same stream as output and input. For instance, a software which record the audio stream send on the output of your sound card. If your sound card has loopback you can do it directly by setting the sound card output as sound input in your software, if not you have to manually loop the signal back to the sound card input socket.
What you need for T/A calculation, is that your input device (the USB mic) and your output device (your sound card) both use the same clock signal. As far as I know, it is impossible with the UMM-6.
And for that you will probably need to buy a sound card that have both the output interface needed (RCA, SPDIF or TOSLINK) and the input interface (XLR with phantom power): usually when both are on the same sound card (internal or USB), they share the same clock signal.
The goal is that the output sound stream (send to your sound system) and the input sound stream (measured by your mic) have always the same time reference, so every mic measurement can be compared one to another: on all measurements, the same sample number correspond exactly to the same output time.
It's easy to test if your hardware use the same clock: put the mic in the car, and measure a full swipe. Wait a minute, and without touching anything else, measure a full swipe again.
It the first maximum peak impulse on both measurement are on the same sample, you're good to go. If not...