Usind zen with an existing member table

We've got an exisitng site with over 10,000 members and now are looking at adding zen-cart into the site do handle a new ecommerce thing we're adding in.

Initially we we're just going to make users sign up again to buy stuff, but that's clumsy and certaining not going to make the users happy -- has anyone dealt with a situation like this and what did you do?

At the moment we're looking at running two member tables, (the existing one and the new zen one) so that when someone joins on one or the other the new member entries are made in both, but it's rapidly becoming a mess!

Any tips much appreciated.