I might be totally crazy but I could swear that the soda siphon we used at the coffee shop where I used to work could be opened up and refilled with water without having to put in a new charger. I honestly remember refilling the water several times a day, but replacing the charger only occasionally. Imagine my surprise when I bought myself an iSi and it has to be recharged every time you open it - but it seems from my research that that's how they all work. Am I totally crazy, or is there such a thing as a soda siphon that can be refilled without having to put in a new charger? It's quite possible that I'm totally wrong; my memory has never been very good.
If you were able to inject the water into the siphon thru the outlet, injecting the water before the siphon is completely empty, then you should not need to refill with the CO2 charger, unless the CO2 is at a very low pressure.
"Tell me what you eat, and I will tell you what you are" Anthelme Brillat-Savarin