More batteries will probably fry the circuit, so there won't be an extension tube. Two 18650 batteries should work but I don't think any extension tubes for two 18650s were made.
Your best choice is probably to bring extra batteries or run the light on low when high mode is not needed.
No extension tubes, yet. If there's enough interest I'm sure Tiablo would come out with one. The A9 can handle 2.7 to 10V, so two 18650 would be no problem. I have an A8S, so I'm stuck with a single 18650.
The only Tiablo type light to have a (promised) extension tube made for it so far will be the MRV. PEU made a 18650 tube for the D-mini which turned it in to a 3 hour non-dimming light. 2x18650's would be a great LED SF M4 type light. We can only hope more manufactures jump on board.