Converting to a normal group chat is currently not supported. When the meeting ends or cancels, all chats will continue to be preserved and members can still be added. To get this feature more quickly, I have found feedback in Teams UserVoice, and you could vote for it.
Feedback: https://feedbackportal.microsoft.com/feedback/idea/eb8972bb-82a6-ed11-a81b-002248519701
Many features of our current products are designed and upgraded based on customers’ feedback. With requirements like this increase, the problem may well be released in the future. Thanks for your understanding and support.
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