This is kind of tricky currently 
I know our team is working on some module improvements over the next few months.
setIn can only work within the module where the variable was defined. It sounds like Module A has the variable, so you can call setIn only inside of Module A.
You can have another variable that gets updated on Submit of the form inside Module B. That variable can be an output of Module B.
Then, in Module A, you can use this approach to watch for changes to the output of Module B & then call setIn. Here's some screenshots of how setIn works.