Use Domain Mapping with different external URL

If those are separate installations of WordPress, you would have to import them into your multi-site installation, not a trivial task. I can do it, it is just 2 dozen complex inter-related database updates, where if you make a mistake you have to start over… (You have a complete backup of your Multi-Site, right?)

Or, create a new site in WordPress Multi-Site, copy and paste the text into pages on the new site, manually make all the plugin, widget, and page settings for each one.

I have not yet found a plugin that will import a WordPress site into WordPress Multi-Site while keeping all the settings.

Or, I’ve heard there are plugins for making administering multiple separate single-site installations easier. If your sites are too large to easily copy page by page, this would likely be the simplest approach.