Accepting full paper is easy. Simply go to Submission type and create a new type, set a deadline and select "Accepted abstracts" in the "Valid for" field. 

Then, go to Form builder to create fields for this submission type. When creating the field, select the abstract submission form to display this field there and then select the submission type for which you're creating this field.

So when your author selects this submission type in the submission form, he/she would see these fields that you created for this submission type. Please note that if you set the valid for to accepted abstracts only, then only those authors would be able to see and select this submission type whose abstracts have been accepted.

Here's a video tutorial for the same:

You can also receive automated notification as an admin whenever someone submits the full paper.

To enable this, go to Automated notifications, Click on Create New to create a new email notification, enter your content etc, scroll to the bottom and select the abstract submission form from the dropdown, then select the field (used for full paper submission) which is when submitted would trigger this notification. Finally, select the admins to whom you want to send this notification. Then, click on Save. That's it.