Mapping Entity records from Lead to Account in N:N relationship

Hi every one, 
I have created N:N relation on lead with a custom entity, same on Account entity N:N relation with that Custom Entity, when i Qualify the Lead those Custom Entity Records created on Leads should show come in the Accounts entity's related items (in the navigation area). What i have read is N:N relation does not perform mapping, how can i achieve this task? Please advise 
Thanks in advance