Unlocking the i5-6400s multiplier with a i5-6600k microcode flash is possible?
Can somebody help me in that i have a MSI B150 PC mate motherboard and the bios flash files i can download for that motherboard in the official msi site isnt even comopatible with uefi for example...
I might even pay some money for the helper

Not that I know of, that is not possible. I can't find i5-6400s (So assume you meant i5-6400), if there is 6400s can you link me to a CPU-Z validation, or Intel or CPU-world page so I can see CPUID

Both of those CPU's have same CPUID anyway (506E3), if you meant i5-6400, so use same microcode and it would already be in the BIOS since both are on the Compatible CPU list.

Here is latest BIOS (v.12.d) from MSI
http://download.msi.com/bos_exe/mb/7971vCD.zip

What is it not compatible with exactly? It is UEFI BIOS (Aptio V), not sure what you mean

Attached below is modified BIOS with updated CPU Microcodes, not meant to unlock any multiplier, but give you latest microcode is all.
506E3 - Original version BA (4/9/2017) >> Updated version C2 (11/16/2017)
906E9 - Original version 70 (8/9/2017) >> Updated version 8E (3/24/2018)