To add music files from other folders to your Xbox Music collection, simply add that folder to your Music library:
- Open Windows Explorer (Win+E) and navigate to the folder in question.
- Right-click on the folder and select "Include in library" → "Music".
- Done!
This can also be done from within the Xbox Music app itself:
- While in the Xbox Music app, open the Charms bar (Win+C) and click on Settings.
- Go to Preferences, then click on "Choose where we look for music on this PC".
- In the new dialog that pops up, click on the + button, which brings up a Metro-style folder chooser.
- Find the folder where your MP3 files are stored, then click on "Add this folder to Music".
- Done!