How to install Windows ISO on a single partition of external HD on Mojave?

What it says on the tin. BalenaEtcher straight up doesn't offer the option to select individual partitions and UnetBootin... well, straight up will not open. I click, it'll ask if I'm sure I want to open it, and then the icon will pop in and right out of the dock. Also, what I'm doing this for is to attempt installing Windows 7 on a partition of the boot drive and then upgrade it to 10 from inside the install itself so I can 1. Work with a sub-4GB ISO file which apparently makes software happier and 2. Upgrade from pirated to legit Windows. There very possibly may be flaws with this plan (especially since the Boot Camp drivers in another partition of the drive in question are made for Windows 10) so if there are feel free to let me know.