You change some code so that a knife skin has range and damage of a firearm.
You can do the same with any weapon,or Med pac,map,radio,ammo pac.
Passing thru walls with bullets or yourself is via the map editor...your map has no solid things for you, but those trying to shoot you have the "normal" map so things are solid for them.
Map editor comes with the game.
Tools for finding the code for weapons are in our download section.
This is a "layman's basics" that apply to basically any game.