Just yesterday, this video popped up on my YouTube feed and it now looks like that custom levels can be made for Indiana Jones and the Infernal Machine. The guy who created the level apparently holds the speedrun records for both Infernal Machine and Emperors Tomb.
The video showing off the level and also the download for the level with instructions on how to get mods to work on the game
https://github.com/Jones3D-The-Infernal-Engine/Mods/tree/main/levels/sed