When you use the Spotify desktop client, any music files found in your Music or iTunes libraries are automatically added to your Spotify library. If you have music in other folders or sources and want to add them, you can do so through the client's interface as part of the Settings menu.
Adding Local Files
Adding new files from your computer to your Spotify library takes place entirely within the Spotify desktop client. While the process isn't difficult, the option can be a bit difficult to find without knowing where to look.
Spotify only supports local files saved in the MP3, MP4 and M4P formats, as well as M4A files if you have Quicktime installed on your computer.
Click the Menu button and select Settings.
Scroll down to the Local Files section and click Add a Source.
Select the folder containing the music you want to add in the Browse for Folder menu and click OK.
Toggle the switch for your new music source to the green ON position if it isn't already.
You can toggle the switches between ON and OFF for all your music sources to tailor which local tracks make it into your Spotify library and which ones don't.