F5 Radius Authentication for admins

I want admin user authentication should be done locally and rest of users should through radius. Is that doable?

If a remote authentication method is specified for system user accounts, the BIG-IP local database still authenticates the system maintenance accounts mentioned above. This ensures that if the remote authentication device is unreachable, the system maintenance accounts can still access the BIG-IP system.

sol12173: Overview of BIG-IP administrative access controls

If user are on both local and radius and radius server is unreachable then user would be able to authenticate locally?

no

Also for permissions, can I use local user role? I mean radius is used only for authentication and for permissions local role group.

yes (radius server is used for authentication but local user setting is used for authorization).