modifying initrd

[BRINER Cedric <work@infomaniak.ch>]

hi,

how can I change the file loadmodules in the initrd.img to add the 
following line:
modprobe -k  aic79xx > /dev/null 2>&1

what is the procedure because
mount -t cramfs initrd /foo
edit /foo/loadmodules doesn't seems to work

Cedric BRINER