Bhavika, User Roles and Mandatory approval are two different features which are not connected to each others.
Role can be given manually by the site admin after a user joined the site only.
The Mandatory approval feature is checked during the join process. The system checks if this feature is enabled, the user's profile goes to the Unapproved profiles list and do not show up on the site.
The requested changes are possible, but since they might trigger the core files modification, it cannot be done as a plugin and requires the code modification.