Have seen stardard end caps that have been converted to bulk fill. the standard end cap has a knurled collor held on by a screw. Can remove the screw, remove the knurled collor, drill though the end cap length wise (about 1/8" seems to wrok fine), drill and tap the upper end of the hole for a fill nipple (one of the paintball valved HPA type nipples). Drill the knurled cap slighty larger so it will spin on the fill nipple (the knurled cap is just there to pull the central part down into the tube as you screw it on, it doesn't seal).
But unless you are set up to drill and tap and have a fill nipple at reasonably cost, it's not a good option. But if you are able to do that, would only be the cost of the fill nipple.
Drilling the cap removes the piercing teat on that end, so it could still be used with 12grs. but only the one down into the tube would be pierced (by the valve's peirecing stem).
