IA02 task list how to validate fields usr00 usr01 ?

Hello, I'm Diego from Argentina, I have entered in a new project and I need help with something...

I've been working for a couple of weeks trying to validate usr00 and usr01 fields from the tasks list in IA02 ...

The thing is that i could not find an user exit to put my conditions... I need something like a field exit.

Any ideas? thank you very much for you time!