Is it possible to effectively use a tado extension kit to replace a three channel controller.

I have underfloor heating downstairs and radiators upstairs. I also have a hot water storage tank upstairs. The timings associated with the firing of the boiler are managed by a three channel controller (Horstmann ChannelPlus H37).

I have installed tado smart thermostats in each room downstairs, set the 'downstairs' channel on my Horstmann controller to permanently on and these work well based on the schedules I have created in the tado app.

I have installed tado smart radiator thermostats upstairs on all the radiators. Again these work well with the schedules I have created in the tado app - HOWEVER - upon setting the 'upstairs' channel on my Horstmann controller to permanently on, in anticipation of the tado radiator thermostats managing the call for heating, the 'heating' actuator for the upstairs area immediately opens and starts the pump next to the boiler. This actuator remains open and the pump continues to circulate water from the boiler to the radiators irrespective of whether or not any of the tado radiator thermostats are calling for heat or not. While this 'theoretically' works, in that the radiators will only actually heat up according to the schedules I have created in the tado app, the heating pump remain permanently on. For example, during the night when I have set the tado radiator thermostats very low (13), the pump keeps on pumping all night.

It had been my intention to continue to control my hot water needs using the third channel on my Horstmann controller, however I have now also purchased a tado extension kit and would like to install this in the hope that I can configure it such that the heating pump only comes on when the radiators are actually calling for heat while it also controls my hot water demands.

Tado have indicated they do not have instructions to replace a three channel controller with an extension kit and have suggested I contact a local electrician to change the wiring.

Has anyone successfully 're-wired' the actuators in their system such that a tado extension kit can replace a three channel controller.

Any advice/guidance greatly received.

Having a quick read of their online page, no specific instructions unless you are registered with them, it would appear that you would need the extension kit for it to work correctly as you have a programmer, so all the previous times you thought it was correct it was wrong? 
I would imagine that the extension becomes the wireless interface and the valves speak to the extension box. 
given that the back plate is a standard heating backplate is have thought that it’s more or less a straight swap for programmer, but I would need to confirm with the wiring diagrams that are with the extension box, so don’t go just replacing it unless you can afford to buy another. 

Sharpened, thank you for your response. For clarity, the tado extension kit is effectively a smart two channel controller. Therefore it cannot simply replace my present three channel controller. 
My hope is that I can simply ‘short circuited’ my existing first channel, meaning my downstairs underfloor heating will be permanently on - safe in the knowledge that the tado thermostats in each of my downstairs rooms (which are hard wired back through the controller to their respective actuator) already work satisfactorily, switching the underfloor heating on and off as determined by the schedules I have set up in the tado app. This will then allow me (I hope) to rewrite the connections for what are presently my second and third channels (upstairs radiators and hot water respectively) so that they become the first and second channel on the back plate and therefore can then be automatically controlled by the tado extension kit. 
I was hoping that someone with more experience/knowledge than me, could let me know if this is possible or not.

BTW, I am registered with tado as a user of their equipment, but even so, they do not supply/provide instructions for implementing the tado extension kit in the way I want to (i.e. replacing a three channel controller).

Thanks.