Importing phpBB3 entails providing me an .sql dump of your phPBB board and I convert it internally to the latest build of xenforo and give you back a xenforo .sql dump you can use on your xenforo files.
It includes the forums, threads, posts, members, private msgs, and a few other basics.
Installing a mod for xenforo is fairly simple. You download it from xenfans.com or xenforo.com and upload the files to your library/ directory and import the .xml that comes with it. And that's it. Well, there might be some addon options you could enable/disable or configure.