There is no need to mess with the existing bootloader of NT. The new ( chain- ) bootloader is installed as a file on the NTFS. The existing NTLDR is the one responsible for running 'grldr' - only if the user menu select it or if is a defaulted selection. As you can see from boot.ini.

To boot NT :-
NTLDR ---> (XP/Vista/Whatever...)

To boot KNOPPIX:-
NTLDR-->GRLDR (grub)--> KNOPPIX