Not sure what you mean by “merge” or “parts”. Do you mean create stereo pair? 

No I mean create a room with an arc ultra and two play 5. But for the moment I have two rooms one with arc ultra and one with play 5.

Thank you

If the Play:5’s are a stereo pair, separate them. Then go to settings for the room with the Arc and scroll down to “add surrounds”. It should offer you the 5’s. That assumes they’re Gen 2. 

Thank you but it is an arc ultra and it is not compatible with all play:5. See here https://support.sonos.com/en-us/article/surround-sound-guidelines-and-limitations?product=arc-ultra

My apologies for missing that. There’s your answer then: it can’t be done