Is it possible to merge multiple organizations within Zendesk Support?
No, it is not possible to merge organizations within Support.
However, if you are on the Professional or Enterprise plan, you do have the option to add users to multiple organizations. For more information on this feature, see the article: Enabling multiple organizations for users (Professional and Enterprise).
Alternatively, create a new target organization for your users. Once you have the target organization setup, run an API call that updates the desired users organization to the target organization. This update doesn't change the organization on any tickets that are in Closed status. For more information on using the API to update users, see our developer documentation on Create or Update Many Users.