It is possible to link them to your Openhab software if you buy a Z-Wave controller - Aeon Labs Z-Stick Gen5 or similar device.
Then you could possibly set up a direct association between them or, more likely, control them through the Openhab software and the Z-Wave binding.
Without a Z-Wave hardware controller, these devices will not be included together in a Z-Wave network. You cannot just buy Z-Wave devices and have Openhab control them.
To answer your question, I do have a Z-Wave controller: I have a Rasperry Pi with a Razberry card on which Openhab 1.8.1 is installed. Im using it so far to control switches and Air conditionning with more or less success.
Practically then to control the thermostat and boiler controller, how should I do?
1- de-link them (the thermostat plays currently the role of the controller)
2- add them to the Z-Wave network controller by my raspberry pi
3- then how should I ensure a command from the thermostat is sent to the boiler controller? I mean in details… Im not an expert at all…