Option to add instead of replace role

[unode]

Currently, if a user is successfully mapped to a given role, any other roles given to the user are removed upon login.
Could an option be added to append the mapped role instead of replacing any existing role?

An example scenario, a user has been given the "Editor" role through wordpress but SAML doesn't contain this information and returns only a "Subscriber" role. At the moment, any role granted through Wordpress becomes effectively null.