Can GIRA radio receivers be added without altering the existing installation?

http://www.gira.com/en/produkte/funk-bussystem.html

andrefff wrote:

Interestingly, a great many components are compatible with Berker’s higher-end models, yet the price is half as much.

The price of Berker’s higher-end models, those manufactured in Germany, is comparable to Gira’s.
The lower price of Berker in Poland applies only to the budget ranges produced in a Polish-????????-Chinese joint venture.
And yes, it will be cheaper. Gira uses, for example, Rutenbeck RJ45 sockets, whilst Berker uses a Chinese knock-off. The only visual differences are the presence of the ‘Rutenbeck’ inscription and the category engraving. In the original Rutenbeck, the engraving is only applied if the socket meets all the manufacturer’s standards. In terms of connection and ‘transmission quality’, they are almost identical. ‘Almost’ makes a huge difference.
The Chinese ones are not subject to any manufacturer’s testing.
The more expensive (German) Berker will also feature Rutenbeck, at a slightly lower price.
The same applies to RTV-sat sockets. Gira, Hirschmann, Berker (not German) – you never know what you’re getting. The German Berker is also Hirschmann.

As for KNX equipment, there is practically no difference.