If you want to take advantage of CiviCRM's "membership management" features (e.g. CiviMember) - I think you have 2 choices:
1. Use CiviContribute / CiviMember online membership signup and renewal forms INSTEAD of UberCart for "self service" member signup / payment.
(check out the Membership section in "Understanding CiviCRM" - our new online book - for background:
http://en.flossmanuals.net/CiviCRM/MembershipPlanning)
2. OR you'll have to implement some PHP code in a separate (custom) module which creates and updates the CiviMember membership records when the UberCart transactions are recorded.