Create Part On demand from CAD Form

Inside CAD Form you could use custom button that triggers a client event and maybe also server event (depends on security) that than add the parts and the Part CAD relationship.

Google for 

1. aras buttons in forms

2. Create new item via Method

If you need more style - use a CUI element.

I will try and Let you know.

How about Relationship Approach, any idea

I will try and Let you know.

How about Relationship Approach, any idea