Migrating an existing site into my MS network.

I have installed a third-party classified ads plugin and accompanying theme, because, as much as I like the classifieds plugin from wpmu, it doesn't allow for a free and paid subscription simultaneously. My plan is to offer a limited free classified service, and providing a one or more premium subscriptions.
My problem is that I set up the site before setting up my MS network. It's on the same account and is set in the same directory format as the MS network. But, it isn't included in the MS network list, as such. I must login to it, as admin, separately.
Is there a way to integrate the currently fully installed, free-standing site into my MS network? The site is functional, and I am having some problems configuring a new install within the network.
The site I am using is built with wpclassifieds theme and associated wpclassified plugin. You can see the site at http://theindymall.com/classifieds/