EBS Connector questions

OIM 11.1.2.3, EBS UM/HRMS ER connectors 11.1.1.5

We have the out-of-the-box connectors running OK. Using HRMS ER trusted connector with the EBS UM target connector.

We would like to customize the ER trusted connector such that it will generate a custom username in OIM and the UM connector can then provision that username into EBS FND_USER.

I'm having difficulty determining just how this is accomplished. Currently ER is using person_id as the username. I understand needing to maintain person_id as the key, however how do I change the way it generates the username?

I've noticed that if I manually change the OIM username it will break the link and the ER connector will create a new user when ran again...