Hey dutch how big did you make your WOFF partition?
In doing some quick reading it looks like I just make the partition (Windows has the tools apparently) and d/l W10 in the ISO version and just install to the new partition. Sounds easy right?

FWIW per MS you can't make a newer Windows OS dual boot by adding an older one (like W7) to another partition.
Something about set-up and boot files being overwritten by the older OS.
That shot that down then I stumbled upon the ISO method for W10.