The only way i can think of is to do a license transfer.
That is where 1 person downloads something and then the other person goes on xbox.com and does a license transfer which gives it to them, BUT the downside is that the person who payed for it can't use the maps offline.
FunkyMunky said:
You could download the map packs online and install them onto your HDD but that is highly illegal, more illegal than most things.