Your question is a bit unclear. If you are asking if it is possible to create the opportunity if the account has no linked contact - by default it is enough the have the account linked to the lead to qualify it and create the opportunity automatically. The contact can be different. The created opportunity will have the selected account (in the left side bar of the opportunity page) and the selected contact as well. The contact will appear on the Contacts detail of Opportunity Details tab
I deleted my question, the problem was that if an account with the same name allready exists in the database, the contact is not linked to the account when we qualify the Lead.
i told to my users to use the lead account panel to select the account if it allready exists and to create the account if it is a new account.