@yang2x
Welcome to the Win-Raid Forum!
Since the required insertion of the NVMe module into the BIOS is very easy even for BIOS modding newbies, I recommend to try it yourself according to >this< Guide. If you are unsure regarding the success, you can attach the original and your modded BIOS. Then someone will do a look into them and evaluate your work.
Edit: After having read MeatWar’s reply I have realized, that your old ASUS mainboard has a non-UEFI BIOS. So the Guide, which had been linked by me, doesn’t work with your system.
I am sorry for my misleading advice.
Good luck!