There are online stores who sell devices to help do this.
I DO NOT work for any of them.
I bought a device to connect a regular Co2 tank to my SoadStream.
I now use a 5 Lb Co2 tank that
I bought at a Home Brew shop and pay about $15 to have it refilled.
As for refilling the original carbonation canister,
beware of the valve in the top. It can make refilling difficult.
Look around the web, there are less expensive solutions than $15
for a 14.5 oz of Co2