You can sync your own music files to Spotify apps on all your devices. So you can combine your music any way that you like.
It isn't possible to upload your own music files to the Spotify servers for syncing back to your devices though. That would require extra music licensing, and so extra charges for customers.
Basically no. But you can import your local files and sync them through Spotify to mobile devices.
What do you mean? You can upload your own music to spotify using a label/distributor: