Hi @krisp  welkom op het forum!

Je mag ook wel in het engels typen als je dat makkelijker vindt!   Wat bedoel je precies met een BT WiFi extender?

BT (British Telephone, UK) WiFi extenders are really mesh WiFi devices like those provided by KPN. They look very much the same and I wouldn’t be surprised if they were manufactured by the same company. Anyway, I would like to try to add one of these BT devices to our home network on the 1st floor of a 3-storey house where we already have a KPN WiFi device on the ground floor and the 2nd floor.

chances are the hardware is the same but the software is different. could that be possible?

Ah, that's an interesting question. I don't see a reason why they would not work with our experiabox modem.
I'm not sure if they would work with our SuperWifipunten because there might be some software issues there as Jeroen mentioned. If you already own them I think it's worth a try. If you're about to purchase one you might as well get another superwifipunt as they're slightly cheaper :-) 

Thanks for the responses. When I get some time I will try to integrate the device, but I know that it won’t “mesh” with the KPN wifi devices, but I think it would work.

Thanks again.

You're very welcome, let us know how it pans out!