This feature is not supported, but it’s interesting/useful enough to realize in a future.
When someone assign new role to a user via WordPress UI: add new user or edit user profile pages, WP_User::add_role() and WP_User::set_role() methods are used. These methods fire hooks:
User Role Editor does not fire any custom action when assigns to a user new roles. It uses the same WP_User::add_role() though, so it’s possible to use ‘add_user_role’ hook also.