For this you will need a drop box, Google music account and a PC. Download drop box and Google music and install on your PC. Also download and install app versions on your device.
In drop box create a folder called Google music. This is the folder you will put music in on your device.On your PC link the Google music program to the drop box file you set up. This means when you up load to drop box from your phone, Google music will read the folder on your PC and add it to your Google music collection in the cloud. If you run put of drop box space just delete the already uploaded songs.
Dropbox
Enjoy!