As the title suggest, I'd like to use the transparent PNG files of the canon NPC's from the original as well as this one to be able to be used as slaves. Unfortunately, I can't find them in the game directory, most likely inaccessible. I'd also like to use them for Lilith's Throne. 

Use the following instructions from this discussion however instead of Strive for Power's .import, use Strive: Conquest's .import:
https://itch.io/t/1990896/importing-strive-for-power-characters

As for the PNG files itself, unfortunately haven't seen an online converter, you'd need the original PNG files or import it to Godot and export it out as PNG.
I'm not sure if the devs allow using it for outside their games however it's up to you. Good practice is to ask first~