Is it possible to convert a running single farm instance (SharePoint 2016) into a mini role instance and then add new server to this farm?
you can do it. Here is MS documentation about it.
How to change a server role:
To change a server role by using the Central Administration Web site
- Verify that the user account that is performing this procedure is a member of the local Administrators group.
- On the Central Administration web site, click System Settings.
- On the System Settings page, click Convert server role in this farm.
- On the Role Conversion page, in the New Role area, click the drop-down box to select the new server role for each server to change.
- Click Apply.
To change a server role by using PowerShell
1. Verify that you have the following memberships:
- securityadmin fixed server role on the SQL Server instance.
- db_owner fixed database role on all databases that are to be updated.
2. In Start find SharePoint Management Shell.
3. Run command
Set-SPServer -Identity <server name> -Role <server role>
Hope it helps!