You can easily add your external media libraries or folders to FCPX's built in media browser in just a couple easy steps. This will allow you to browse folders and libraries on your machine through the Final Cut Pro X interface, without having to tab over to a Finder window, look for the items you want, drag them in, or fuss around with the import window.
1. Simply drag whatever folder ("Test Folder" in this case) containing your desired media library in to the Music and Sound browser.
The folder will automatically appear in the Sound and Music browser from now on. If the media folder being referenced is on an external drive and the drive is disconnected, then the media will be unavailable, obviously. If you had already used media from the added folder, it will have been copied to your FCPX Event, and will not go offline, even if the drive was disconnected.
To remove any added folders, simply right click on the folder in the Sound and Music browser and click "Remove Folder."