Hello Craig,
I fear there’s no way to do this automatically. You’d need to download a csv from your ecommerce plugin with all those guests customers (along with their email address), format that csv with a list number (you basically need to maintain only the basic info, like name, address, and list number – remember that it must be previously created), and then upload it using our import feature. It will update the existing matching subscribers by assigning them to the specified list number. Maybe perform some tests with a couple of subscribers to check that everything is properly working.
Michael