Install Windows 7 On MacBook (x) 2015

You can install unsupported operating systems such as Linux or older Windows versions by not using the bootcamp assistant.

Open disk utility and create a partition, format it FAT. Then reboot whole holding alt/option and boot from the install-disc (CD, DVD, USB drive, ...) and install as usual.

Now comes the tricky part. Since the OS is not officially supported by Apple your on your own regarding driver-install. You'll be able to just use the regular drivers from the vendors for things like audio and wifi, for apple specific drivers, you can use the drivers that are downloadable by boot camp assistant. They single drivers are usually easily accessible in the package, you just can't use the Install-Everything App that Apple bundles with them.

Alternatively, use virtualization is a good option.

For those of you who:

1. Have a 2015 model of MacBook and does NOT support Windows 7
2. Want to take advantage of the free Windows 10 before July 2016

Bootcamp supports Windows 10 now, so I just spoke with Microsoft and gave them my Windows 7 key... and they provided me a new Windows 10 key to use.

Just adding this to the post since I fond this in my search for resolution.