I can offer my comment from an issue over on github:
Just in case someone comes across this issue and wants to dig a pit or “place” dirt for other reasons (without cheating) in the underground (or above ground) before other methods are integrated into the game:
If you open the construction menu and select “Build Roof Over Dirt Floor”, you can build diggable dirt at a place of your choosing (needs support walls or an existing roof on at least two sides if built above ground).