Thanks @Fernando. I have followed the steps of inserting the NVMe module (NvmExpressDxe_5.ffs) using the MMTool (AMI Aptio UEFI MMTool v5.0.0.7) and saved the result in a new folder (P9X79-ASUS-4801-Mod.ROM.fd).
This is where I’m tripping up… " Don’t forget to re-insert the original capsule and to save the modded BIOS as *.CAP file according to >this< guide written by MeatWar."
I have looked at that thread by MeatWar and cannot understand what I need to do. Is it as simple as renaming the file ‘P9X79-ASUS-4801-Mod.ROM.fd’ to ‘P9X79-ASUS-4801-Mod.CAP’?