As I wasn’t able to find much info about Telenet's powerline adapters when I was looking to add some adapters to the existing network, I thought it could be useful to share this

According to Telenet technicians, you should be able to add almost any brand of adapter to the existing network. Although they didn’t guarantee it.

So I took a risk and ordered some additional adapters from D-Links 500AV series (D-Link Kit DHP-P501AV SchuKo Powerline AV Kit (500Mbit)).
I was able to add them successfully to the existing network.

So it is possible to add powerline adapters from a, different brand, even 500Mb adapters (as the onse from Telenet are 200Mb).

Adding the new adapters was pretty easy;
Plug in one new adapter, press and hold the “security button” for 2 seconds on one of the Telenet adapters (the only button on the bottom), the power led will start flashing. Then you have about 2 minutes to press (a,d hold for 2 seconds) the security button on the new adapter.
The new adapter will try to connect,to the existing network, and the, reboot, after it’s rebooted, you should be able to use the powerline.
If you have multiple adapters, just repeat the steps for the next adapters.