I'm thinking ProScan may use the .txt file extension for scanner programming files it creates.
If you created your 996T files with ProScan, then you could use ProScan to program these files in your 996XT.
Also, just going through all the options, ARC XT also has a utility to open a 996T file (created with the ARC 996 program) which can be programmed to the 996XT.
Cheapest option, however, is ks3jiz's suggestion - using FreeScan.
Since the 996XT has greater "capacities" the 996T data will fit in the 996XT, but not the other way around. (You can put the contents of a quart jar in a gallon container, but not, necessarily, the gallon container in a quart jar).