Sorry, which many-to-many relationship are you talking about?
I don’t think you “convert” a Lead to an Opportunity. Conversions are between person records, so: from Target to Lead, and from Lead to Contact. You can skip the Target concept if you don’t need 3 levels.
The Opportunity, on the other hand, is not a person, but a (potential) business deal, which is related to that person.
Well If I convert a lead I have the possibility to create a new opportunity, related to that lead and contact. But how do I use lets say the contact’s phone number in the opportunity workflow?