You don't have to have a different ISAKMP key if you don't want, but the PIX can certainly have it. Just do:
> isakmp key address netmask 255.255.255.255
> isakmp key address netmask 255.255.255.255
Of course if you don't want a different key for each, you can just do:
> isakmp key address 0.0.0.0 netmask 255.255.255.255
and the PIX will use for all the satellite offices.