Currently new folders can be added by:

• [it will happen automatically if "Listen for file changes..." is enabled]
• "Rescan for new and changed music files"
• "Clear and rescan"
• If it's a separate folder to the existing music library, adding it as a new folder in the settings page.

If you don't want listening enabled, the trouble with the second two is the length of time they take for a large library.

It would be nice to directly add a folder of music files.