Createing a custom compeduim entry

To elaborate I have some home brew items and adventure gear that I would like to have in the drag and drop format that the new compendium supports however am unsure how one would add it. My investigation on how it works is with Json files. Again not quite sure if this is something that is not currently added or not.. however I have experience with Json and html so I'm confident enough to sit and work on adding them in just need the how one would add to it. Thanks!

Custom compendiums are not supported atm.

Ah would one be able to make an API script to import the functionality of one? I ask as I have not messed with it too much however I am fully prepared to sit and dev one :)

Certainly, I believe the 5e shaped companion has functionality for this, and I have plans to add similar to the pathfinder companion script (at some point, don't ask me for an ETA).