I want to convert or qualify a lead into opportunity, but the lead has some self-define fields and I also want these self-define fields to go into the opportunity automatica
You can do by editing the mapping of the relationship.
You can define the source field and the corresponding target field
As Guido said, you just need to modify the Lead- Opportunity Relationship. When you go to your solution, go to the Lead entity and to the relationships tab, there look for the Opportunity one, when you open it you will see a Mappings tab, there you can define which fields will be populated.
However please have in mind that this will only populate the fields on Create of an Opportunity!
If you want to also update automatically that opportunity when you update the lead you will need a plugin to do that.
Cheers,
Mario