Is there a way to add more default values to the grant entry defaults?

There is no way to include additional values in the Defaults screen, but you can try the following instead:
 

•  Create an Application, perhaps using your organization's Profile or a dummy Profile as the Grantee and leave it as Status = Open (or check the On Hold box) so it will never get posted by mistake.
•  Use that as a template and fill in all the values you want as default information on the Application.   Save.
•  Click the Copy button to create a new Application based on the template.   Change as needed before processing.

  Multiple templates can be created for different types of Grants.