Sync IDV workforceID to Google Apps Employee ID field


I know this happened been asked before in the forum, but I didn't see any solution for this. 

I am trying to sync IDV's workforceID to Google Apps's Employee ID field. I had tried many attributes as suggested from some Googling and from this forum but no success.

Only indication is to do with ExternalId attribute, but I had no clue on how to do it. 

Anybody had any idea ?