You need to connect to your NAS with a SSH tool like "putty". Login with the user "root" and your password.
Once logged in you type
cd volumeUSB1/usbshare/boot/grub/
"volumeUSB1" could differ from your configuration. I use a USB-Stickt to boot with gnoboot.
Next:
vi menu_alpha.lst
This opens the file "menu_alpha.lst" with the vi editor. Within the editor you put the cursor where you want to edit, then press "i", wow you are in "edit-mode". Make your changes and afterwards press "Esc" to quit "edit-mode". After this type ":wq" which will write the changes to the file and quit the editor. Done! Not sure if you need to reboot after the changes, I did...
BTW @quantum: You are my personal hero of the week! Could you share some light on the issue why "sn=B3JN00001" is needed?
Greetings!