Competitors When Deal Lost

Competitors When Deal Lost

I would like to add a lookup field to deals that is only visible and only required when the deal is closed and the reason is marked as "Lost to Competitor" This lookup field would use the competitors module to allow the salesperson to link the deal to the competitor they lost the deal to.

I can handle the visibility and required based on the stage using workflows, but my problem is that competitor is not available as a module when creating a lookup field. Is this possible?

Thanks