Hello Sandy,

First, you will want to restore your mail folder from the backup to say a new folder on your desktop.

Directions on restoring data with iDrive click here.

Directory to be restored to a folder on your desktop from your backup ~/Library/Mail/

How to import a mailbox into a secondary folder click here.

Once you have your old data imported to a secondary mailbox folder, you can now drag and drop missing emails from the imported folder into your primary inbox.