Wishlist registration vs subscription purchase

I have been testing a membership site setup with Wishlist Member, WP 3.0 and Simple:Press. It has taken quite a bit of work to get all of the various pieces together (the various roles, member levels, etc.) but I seem to have it all working. All except a major component.

I have contacted WLM support three times. Their first response took three days. The last two have been within a day. They have been pleasant but the bottom line is that I still do not have my answer.

The site:
Has blog, pages and forum.
Has a free Registered Guest Level with limited content.
Has four separate (paid) membership groups each with three levels of membership.
I have all of that set up and it seems to be delivering the right content to the correct memberships.
The registration page for the Registered Guest (free level) seems to work fine.

Here is the problem:
Once you have registered as a Registered Guest I want you to able to purchase subscriptions to the other membership levels under that same account. So, after registering you may then decide to subscribe to Group 1 Level A and Group 3 Level B. Someone else may want to join Groups 2 and 4 both at Level C.
If you direct them to the “registration” pages for those groups and they click on existing member and fill in thier user id it adds the membership without a charge.
If I direct them to the subscribe button, when you click on that it seems to be settings up a whole new account.

WLM support says it should be doable (the only limitation being that they need to subscribe to one group/level at a time as opposed to all in one transaction which is no big deal), however, I sure have not been able to figure out how to accomplish it.

How can an existing free member subscribe to additional (paid) content under the same account/signin?

Can I lock (or eliminate) those other registration pages so that they can not sign up to additional groups without paying?

Thank you for any light you can shed on this.