Yup! Like I said, the Migrate DB Pro Multisite extension handles it like a champ. You’ll need to setup the multisite first, of course, and determine the primary domain (MS must have one). From there you make sure all the themes/plugins that are used on the single sites are uploaded to the MS. At that point it’s recommended that you pull (as opposed to push) each site into the MS one by one.
Multisite isn’t really all that complicated. You really just need to make sure the themes and plugins are over, Migrate DB Pro will handle the rest —that being pulling over the media (make sure you have that extension as well), adding the appropriate tables to the database, and adding the site to the
wp_blogs table. If anything in there doesn’t work, you can look over the database yourself. Probably the most complicated parts of merging into a multisite is dealing with users and discrepancies between plugin/theme versions.
Depending on your level of familiarity with a single WP installation and the number of sites you’re merging, it could either take an hour or two, or the whole day.
If you get stuck/lost feel free to shoot a question in here.