Windows 7 on MacBook Pro 2015

Hello.

The windows compatibility list on Apple's website is not up to date (Install Windows 7 and earlier on your Mac using Boot Camp - Apple Support). So I mistakenly bought a windows 7 license thinking it was compatible with all macbook pros while it is not supported on the latest model (2015, which is not on the list). So now I'm ****** and stuck with a version of windows not supported by bootcamp. I tried hard to install it anyway with no success thus far. In particular, I made a dmg of windows on an old macbookpro with a dvd reader. Then I tried to copy it on a USB key using either terminal and "sudo dd ... bs=1m", or using unetbootin. In both cases windows is succesfully copied on the key however this key is not detected by the startup disk manager. I don't see it in the preference panel, and it doesn't appear when I boot and press the option key. It doesn't boot on an older macbook pro either.

Does anyone know how I could make that USB key bootable? What am I doing wrong?

Thanks a lot.

Baptiste