Can a Defaults line be added via the Sudo GPO in Group Policy?
This is not possible through the Sudo GPO in Group Policy.
A potential workaround is to use the files GPO to copy an edited version of the sudoers file to the client.
Unix Settings | Quest Authentication Services | Client Configuration | Files
For example, use the GPO to take a copy of an existing sudoers file, edit it and then use GPO to apply the file.