Your English is excellent. 

Sonos does not offer a way to add more speakers to the Home Theater system. Other than adding a second Sub, you’re at the maximum allowed speakers. You could swap out different surrounds, but I think you’re already using the best available speakers there. 

No, another sub is all you can add to the home theatre setup.

You could add another pair of speakers and group them with your home theatre setup but they would just be stereo and would potentially work against your home theatre by reducing the soundstage/surround. Plus, as all your home theatre speakers bond with their own 5ghz network, any speakers outside of that may well be out of sync.

Thank you for this quick response, Airgetlam. I will just buy an additional subwoofer then. It's a pity we can't add more speakers; it would have allowed me to blow the walls of my living room apart :)

Thank you also for the response, Rhonny. I hesitated for a long time back then between the 2 Era 300 and the 2 Sonos Five. Now that I can afford it, I would love to have 2 Sonos Five in front to deliver powerful sound and be even more immersed in the soundstage.

Yes, it would be great but each speaker needs to receive a certain signal from the TV/movie content to create the wide soundstage - so with 7.1.4 already covered by your home theatre, if you then added stereo Fives in the front, they would play all the sound in just those two speakers so it would make your actual surrounds and soundbar seem less impressive…

Thank you for these details, I understand. Maybe one day, an update from Sonos addressing this need with an adjustment and balancing of the soundstage could be a good idea.