Approvers & Guest Approvers

So lets say I add approvers to a project, they go through their approving process and they want me to add guest approvers to the same project that they are involved in. Is there a way to add guest approvers and keep the same (existing) approvers on the project without having to retype everyone as a guest approver?   Also when I type in an approver that has an account as a guest approver, their email doesn't auto populate the field like it would if I were adding them as a regular Approver.