Is there a straightforward way to use ee to migrate an existing site to being multisite on WP using ee? I see that you can create new multisite installations easily, but can I transition an existing site on nginx that was not created with ee to multisite?
The easiest way to migrate is always to use a plugin. Most good backup and restore plugins will also change URL’s on restore if you move a site and handle all the heavy lifting of moving your site. There are many and it is hard to recommend any one over another but I have been using wpvivid for the past few months and it seems to work. Check out the following (in no particular order or preference): updraft Plus, Jetpack, Backupbuddy, BackWPup, and Duplicator.