I have just now registered to this forum to say hi and thank you to this community and to share something that I wasn't able to find on the internet.

I have successfully modded the BIOS for my ASUS P8H67-I PRO board for the purpose that we all know here ;-)

Finally, I decided to occupy the empty PCIe with a cheap NVMe M2 (KingSpec brand) SSD from AliExpress.
(link to the SSD product page: h t t p s :// bit.ly/3sOrhrg)

Thanks to this forum, all the information was easily available do the mod of my good old ASUS P8H67-I PRO MOBO myself.

I have used this PCIe adapter h t t p s ://bit.ly/2PV1OOh

I followed the guide on h t t p s ://w w w.win-raid.com/t871f50-Guide-How-to-get-full-NVMe-support-for-all-Systems-with-an-AMI-UEFI-BIOS-388.html up until the step 4.

I have used the UEFITool_v0.28.0 and after opening the original BIOS I could see a message "parseBios: one of volumes inside overlaps the end of data".
But the step "3. Insertion of the NVMe module" completed without any message and there was enough space in the "drivers" volume (arround 200kB) though, so I have used the NvmExpressDxe_4.ffs.

Screenshot:


I was able to successfully flash the BIOS via the EZ Flash tool which is part of the BIOS.

The original ROM was downloaded fro m the official ASUS website (link- h t t p s : // dlcdnets.asus.com/pub/ASUS/mb/LGA1155/P8H67-I_PRO/P8H67-I-PRO-ASUS-0601.zip)

So here I'm attaching the modded BIOS file.

P.S. Thanks @PiGeon, and sorry bro - you see, I borrowed some part of your original text for the sake speed copy/paste. That's due to the fact that our MoBo's are similar.
Thanks!