How to Restrict Search based on the Roles for External crawled sites

I have a situation where the search results have to be restricted based on role

When External sites are crawled, how can we restrict the search results based on roles,

I know that we can restrict the search to a group or set of groups that can contain many users but if the group have different roles and if that group has given access to a web repository search, how can we restrict the document/search access based on roles for the same group?

For Example an Index that has external site as data source and the permissions were set for a group and that group has 2 roles, lets say <b>"Admin" and "user"</b> and the external site have some documents when searched the documents should come up only for the "Admin" role during search, but should not come up for the "user" role

Is it possible to achieve this? Is there a solution?

Any advices are greatly appreciated and awarded

Thanks,

kk