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.