What kind of device is it that you are wanting to add music onto?
In order to get Spotify downloaded/offlined music onto a device it has to be able to have the Spotify app on the device such as an iPod touch/mp3 player that supports internet apps, or any mobile phone, touch pad, or desktop OS that supports the Spotify app on it's system. So just a standard iPod will not be able to offline music on to it, you can use the Spotify app to add local music files only onto an iPod. So are you wanting to add local music files or download/offline music onto a device from the Spotify service?
I feel **bleep** by Spotify because I can't load this music on my mp3 player. I am going to burn your **bleep** for it, Spotify. As soon as my 3 months are up on my premium membership, I am canceling it. Take that you rat bastards, I can screw you too.
Peter, your dumber**bleep**. I don't have to pay for a **bleep** song to put it on an mp3 player. I can download it off YouTube and convert it to an mp3 file for nothing. You don't want people to know that, do you?