I'm pretty sure I've seen this topic covered by Ipstenu and/or Andrea in Multisite support forum. But this demands a in-depth search in the un-friendly search functionality of WordPress.org.
And if my memory serves me well, I think the solution is to invert the method used to extract a site from Multisite.
This method is described in the 12th chapter of the book WORDPRESS MULTISITE 101, by Mika Epstein (aka Ipstenu).
She has another one that's quite worth reading as well: WORDPRESS MULTISITE 110, in partnership with Andrea Rennick.
Don't forget to tip the jar if you find them useful :). Well, they are really useful :)))
I think there are other plugins that do it as well, but am not sure...
The solution I use is a paid one: BackupBuddy. It allows extracting/injecting sites from/to a Multisite in a breeze.
If you follow this path, my suggestion is to:
- export the Single site
- import the backup file into a new Multisite installation
- check that is all working in the basic folder or subdomain address
- map the domain to this new Multisite site (in the cPanel and in the plugin)
- you still will have the original WordPress installation files residing in your server and its database data (case something goes wrong, simply un-map the domain in the plugin and map it to the old folder in the cPanel)
Use InfiniteWP. It is like ManageWP, but free.
With it you access all your sites dashboards (Single or Multisite sites) in one single interface. And if you excuse a little excess, it is damn cool !