You can create new bindings in game through the console, or by writing them directly into the KFInput.ini file. If you make them through the console, you can't set the Control, Alt or Shift flags, or their alternative flags. So if you need those, you'll have to edit the KFInput.ini file directly.
To open the console, hit the ~ key. This is where you can enter all sorts of commands, and I wish I could provide you with a link to all the commands and what they do, but that doesn't exist as far as I can see. So you've kinda gotta piece together what they all are and what they do on your own.
To make a new bind, you use setbind name "command". One simple binding that I have is hitting X to say "Thanks!" to my teammates. You can make this binding by typing setbind x "say Thanks!" into the console. Another useful binding I have is one that switches to my second primary weapon when I hit 5. This binding can be created by typing setbind five "switchWeaponGroup 2|switchWeaponGroup 0|switchWeaponGroup 0".
And that's how binds work. I find a lot of people don't use them, and then complain about problems that can be resolved by just making a couple custom binds.