How to transfer music to iPhone from another computer?

I'm using my dads computer and I have a few songs that I want but I can't transfer them without erasing all the music I have now.
If I log into my iTunes account, I still can't transfer them. I don't know what to do! Help me please.

Added (1). I also authorized the computer I'm using but I don't know what to do from there.

Added (2). I can't transfer purchases because I didn't buy the music. I got it on a free music download website.

Connect Your Iphone to the computer. DO NOT SYNC IT. Right click the Iphone in the tool bar and click transfer purchases.

It is not possible to do what you want to do if you didn't purchase the music from iTunes.

The only way to do this woud be to burn Data DVD's from whatever computer you got the music from originally. Then load the DVD's into Dad's iTunes.

If you try what your thinking, you'll lose all the music on your Phone.

Have you tried selecting Manual Management of Music and video in Itunes? Do that and just drag music file from its folder on your father's HD and drop onto the iPhone music tab in iTunes.

You can try iStonsoft iPhone to computer transfer, to copy songs from iPhone to your computer,
I have used it for two years, very easy to use and powerful!

Google the " iStonsoft iPhone to computer transfer"

You can use 4easysoft iPhone Manager, it supports to transfer files bewteen iPhone and computer.

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