Creating a game using HPL2

[Kein]

Would it be possible to create a game with the HPL2 engine that would be free?

The way you constructed the question implies that all the existing mods are paid. But they aren't, in fact, it is exactly opposite - you can't make a paid mod/game based on HPL unless you work on/with FG as The Chinese Room does.

Also, yeah, you don't have source code to tinker with.