It's easy man. I refill mine all the time and do it for my buddies too.

step 1: remove the o-ring from the nozzle

step 2: carefully clamp the nozzle in a vise with brass jaws or use vise grips but stay away from the o-ringgroove. It's critical that you do not mess up the sealing surface.

step3: pull nozzle out of bottle.

step 4: take a small 1" long drywall screw andjust barely screw it into the check valve below wherethenozzlewas.Maybe 1 full turn after it digs in to the check valve (white round bump)

step 5: clamp onto screw head and pull it out now

step 6: put some crazy glue or loctite cement around nozzle barbs on the outside and reinsert it to the same depth it was at before. Now put the o-ring back on.

step 7: with plastic tank upright, refill with butane bottle.

couple of notes

-Always make sure that the tank you are about to modify is EMPTY!!!
-mark each bottle after modification so you know which ones you have already done
-when refilling, it may stop before actually full. in this case bleed off just a seconds worth of gas to make room for more liquid form

And finally, I take no reponsibility in you performing this modification on your own!!! DO SO AT YOUR OWN RISK!!!