How would one add custom made 3rd party pieces to LEGO Digital Designer?
In the comments you wrote:


  ...For the record, I am willing to consider answers that address
  other software as well. But I currently only have the LDD, so am
  completely unfamiliar to any other software out there. – nijineko


Brinklink offers two programs that could be right up your alley:  


Studio 2.0 - A popular alternative to LEGO Digital
Designer, and

PartDesigner - A side-tweak program to make custom bricks,
where    you're able to export directly into the Studio 2.0 program.