They make a needle that you can get for your grease gun, then you can shoot the grease right into the bearing, pull the race up and rotate then shoot some more in there. Once you're satisfied they are well packed, bag them until you are ready to reinstall.
Like I said above, be sure and get as much as you can into them because that is the only lubrication they get. The gear oil doesn't make it out to them.
While you have it apart make sure to peek in at your axle seals too.