You mean when you play IS + the "TROM for IS" mod? That's someone else's work. You'd have to ask on the IS forum
I would just like to point out that with that mod, you will be missing out some of the latest improvements we included in TROM, and I believe it also does not include any of the new regional units from TROM. (I don't mean to take away from that particular mod, I'm just doing my marketing job for our own shop )