To do this you'll need to create another field for Due Date Reminder. I believe you can make that a calculated field that is equal to Due Date - 7 days. You'll also need to create a workflow that kicks off the email to the desired recipients.
For your Task list, go to List Settings
- Information management policy settings
- Click on Task under Content Type
- Select Enable Retention. Some new text will show up
- Click on Add a retention stage..., which will open up a modal window
- There should be a drop down for Time Period. Select your DueDateReminder field.
- Under the When this stage is triggered, perform the following action: ** dropdown, select Start a workflow then select your email workflow
This should get you what you're looking for.