Ask Non-Registered Users to Register
If I understand correctly, we can't add a web login to an existing member. That means that as we drive membership to renew and purchase on-line, every one will have to create a duplicate record when the register on-line, then we will have to de-dup every member.
It would be better all-around if we could email existing members who did not register (everyone at this point) an invitation to register their existing account. The member would click a special link just for them, and they would add their login and password to their account.