This looks very similar to my monster transformation tutorial, using the states and such. I'll take a look and see if I can do anything. I'm not too familiar with that battle system, but I'll get a copy and see if I can find a solution for you.
Edit: I'm tired so this took some time, but I was able to make this work using the standard battle system, I'm sure you can adjust what you need to make it work with that battle system.
First off, you set up the states just fine. Make sure the interrupt is offset by opp. and that the second state is set to non resistance.
Then create a skill that causes interrupt and also have it call a common event that turns on a switch. Finally set up your troop events to look like this.
Don't forget to set your second state to remove itself after a number of rounds so that it does not stay immune to interrupt.
I hope this explained it and that it works with the battle system you are using.
Edit: Ignore the conditional branch, I put that there when I was testing with two monsters, but if you HAVE two monsters, you can set it up as such