I want to add the Substitute Cards I have in my physical collection to my digital inventory but, the Substitute Cards don't seem to be available in any of the "Extras" for all the different sets that have them. Is there a way they could be added to the site or am I just not seeing them in the card database?

A Substitute card (or double-faced helper card) is an optional substitute for a double-faced card or meld cards, providing the standard missing card back these cards are naturally lacking.

https://deckbox.org/mtg/Double-Faced%20Card%20Placeholder?fromqs=true
these ones?

Some are also under
https://deckbox.org/mtg/Checklist

There is still some that are missing from recent sets