If you are a game developer wishing to add physics to your game, there are several engines out there you can use - Havoc comes to mind as a commercial example, Open Dynamics Engine if you're into Open Source.
Or you can roll your own, as Alex has done.
Putting Pontifex in SimCity (or similar) would be tantamount to saying: I like my Quake skin to be usable in "The Sims", so why can't you make the Quake engine a plugin and have the sims render all graphics with it?
The amount of code integration, graphics conversion etc. would be akin to developing a whole new game.
These topics make great April fools jokes. Unless they're done with the same codebase, different games will usually not be integrated.