Opportunity/Account/Contact Relationships

With regards to the Account/Contact field in Opportunity, currently this can only be tied to an Account or Contact, obviously.  My question is, is there a way to remove the ability to attach it to an Account and instead add Lead as an option?  If I can't remove Account from the equation I'd at least like to add Lead to have three options.  I'm assuming this is done through an N:1 relationship, and I see that one exists currently between these entities for the "customerid" field, however if I add a relationship for Leads how do I then get that relationship to show in the drop-down menu when tying an Opportunity to an Account/Contact?  Finally, the relationship behavior between Contact and Opportunity is Parental however that between Account and Opportunity is Cascading.  Am I correct in my understanding that this means an Opportunity could exist if the Contact it's attached to is deleted but if the Account it is attached to is Deleted the Opportunity will be deleted as well?