Update custom entity as a "related entity"

Hi guys,

I created a custom entity "etest" and have a relationship against Opportunity. It is a 1:N (opp -> etest).

I tried to create a workflow where a new record is created it should update my custom entity (this custom entity already has data and is related with a Lead), but it doesn't appear as a "related entity".

How can this entity appear visible as related or is there a workaround for this?

thanks people.