How to export a single WP site to a WPMU subsite?

Hi,

One of my customers had a single WP site. He went to move it inside my multisite network. What is the easiest and realiable way to proceed? Is there any WPMUdev plugin that does the job? Or shall I use the official WordPress Importer plugin? (and if so, I simply need a .xml file, correct?)

Thanks!
Best Regards