How to share a record in MS Dynamics CRM using workflow

I would like to do the following:

when a Sales person assigns a custom entity (let's call it 'Primary Expertise') to an Opportunity in MS CRM 4.0, the system would share the Opportunity with the user that is defined as the Owner of the associated 'Primary Expertise' record.

I would like to do it automatically via workflow but cannot find the workflow step that would accomplish that. Yes, and I read on some forums that it's actually not possible yet, only via a .NET assembly.

Experience, anyone?