You can pick up the "daylight spool" (solid side) metal reels pretty easily, but beware that they are a tight fit in some bulk loaders. For instance in a Lloyd's style loader, you have to not *quite* screw down the red knob all the way, or the ribs on the inside of the lid will rub against the metal spool and make it too hard to spin.
To do it the right way, you need a pair of rewinds, split reels (a 400' or bigger for the original roll of Double-X, and a 100' or bigger for the 100' spooled down roll), and some more cores for spooling on to. Those 1" cores from your HP-5 won't go on a split reel, so you'll need 2" ones. The above-mentioned method (two nails on smooth wood) can also be used to spool from the 400' roll to the 100' roll (even using your old 1" HP-5 cores) but it's tedious.