Surely it's easier to just buy a longer piece of cable instead of joining it ?? It's just figure of 8 cable with screw terminals to go into at both ends...nothing fancy needed.
On my install the ATF600 is mounted on a wall without a back box where the thermostat used to be so no room for the transformer so I decided to mount the 5v transformer in the wiring centre in my boiler closet, and use the original two wire thermostat cable that runs through the plaster and under the floor for the 5v supply. Works fine even though the wire gauge and length is near the limits recommended by Honeywell. (For voltage drop etc)