While the 5000 is definitely the most versatile bag that EXO currently offers and I don't think you'd regret having one; You definitely don't need it solely for the purpose of increasing water carrying capacity.
Like others have mentioned, load up a couple of HydraPaks (or Droms or whatever bladder) and strap it to the load shelf with the Crib panel. It will carry far better than loose in the pack and will expand your ability to continue to use your 3600 to get by. Between a Nalgene and a 3L water bladder in the pack, plus a 6L+ bladder, there's your 10L of water. I don't think the second Nalgene pocket is a bad idea either.
On another note: I seen your comment about chugging 1.5L prior to heading up for the hike; and while its good practice to pre-hydrate before a big effort, slamming down that much water prior to your hike is simply unnecessary. If you're diligent about staying well hydrated in the days leading up to your hunt, you don't need to pack that extra 1.5L of water (in your stomach) on the way up. Your body is going to upregulate and your kidneys are going to get rid of the excess water you ingest as it sees fit anyways... I would say if you're already hydrated, sipping on a 16-20oz bottle of water w/ some electrolytes (namely salt) on your way to the trailhead; you're more than adequate.