Create new multisite from existing and keep domain?

Yes, you can set up multiple installs. You do need to be careful that you don’t ever create anything in the “parent” multisite (at example.com) that has the same slug as the folder you place the “child” multisite (at example.com/bananas) – so in the bananas example, never create a Post, Page, Category, Tag, etc. with the slug bananas in the parent site.

You could also consider a Multi-Network – which is basically a MultiSite of MultiSites. Most plugins aren’t compatible with these, so if you rely on a lot of plugins that may not be the way you want to go, but it can help ensure you don’t have conflicting slugs that cause the “Do I show the actual folder, or the content at this slug?” conundrum.