How to Convert a User Mailbox to a Shared Mailbox
Convert a User Mailbox to a Shared Mailbox
Go to the Office 365 Admin Center.
On the Active users page, select the user whose mailbox you want to convert.
On the user's page, expand Mail Settings and click Convert to shared mailbox. You won't see this option if the user doesn't have an Office 365 license assigned to them. Go assign them a license then come back to this step.
Click Convert to complete the conversion.
Reference Link: https://support.office.com/en-us/article/convert-a-user-mailbox-to-a-shared-mailbox-2e122487-e1f5-4f26-ba41-5689249d93ba
Updated on: 31/01/2023