Entering bulk test data into Desktop applications

The company I work for has a suite of 5 Desktop Applications, lets call them Application 1-5. We work in Windows. Occasionally there is a need to enter a significant amount of test data in QA, in Applications 1-3, for use in testing Applications 4 & 5. I am trying to figure out the most efficient way to do this. Out of the 5 applications, only one of them has a API Service call we can use, to enter bulk data through JMeter. Also, we can't do this via SQL in the back-end.

We currently use TestComplete to test our Desktop Applications and its fine for that purpose, but its not efficient at all for entering a significant amount of test data, as far as I know.

I have looked into winappdriver as a possibility, but not sure how good it is for that purpose either.

Any suggestions?