The DECT connection between the FritzBox and the FritzFon C6 will certainly work. But how you integrate the whole thing into your Panasonic telephone system, I can't say, sorry.
You will certainly not be able to use all the control and Smarthome functions of the FritzBox (if at all), and in my opinion not all the energy-saving functions or mechanisms for reducing DECT radiation will work. If you want to use the complete range of functions of the telephone, it is advisable to switch completely to the FritzBox.

Yes, I have just connected 2 new c6 to the old FritzBox.I can't say anything about Panasonic.

Thanks for the answer