They "say" it's not possible, but anything is possible, and that's why I'm asking someone who actually writes code.
I can give you a box of tools to fix up my car, but without tires you arent going to be driving it anywhere.
You cant just mod multiplayer into a single player game, multiplayer is closely tied with how the game functions. Only game that has done it to my knowledge is a mod for GTA 4, which I dont think any good (worse than APB gameplay).