Spotify recently introduced a new "My Music" section that allows you to save albums and tracks you find in Spotify to your own personal library. That's great, but if you want to mesh that library with your local files, you need to do that manually.
On the desktop version of Spotify, click on the "Local Files" section on the left side (if this isn't available, head into Preferences and make sure "iTunes" is checked under "Local Files"). Grab any tracks you want to show up in Spotify and drag them over to the "Songs" section under "Your Music". Now, those tracks will show up in your My Music section in Spotify for streaming on mobile and other computers. You're still streaming from Spotify, but this approach means your two libraries become one.