I'll try to explain this as simple as possible.
To Keybind/Marco somthing in game, open the console via ` or ~ should open the console. (Apprently F3 opens it too).
So say you want to bind a key to Roll the Dice. (In this example I'll use the Key Letter J)
To do this you would open the console then type setbind j Say !rtd then press enter. This should make it so you can press your j key and you will roll the dice without having to type it.
Some keys can not be binded to this way and others are more complex then just typing in the key to set the bind.
For example if I want to set the Roll the Dice bind to a right numpad key It would have to be setbind numpadone Say !rtd. This will set your right Number 1 key to roll the dice for you.

There is a way to setbinds without being in game, but it might seem more complex or it might seem easier depending on who you are.
To set binds out of the game go to your Config folder for KF2.
Example - C:\Users\Fluga(Me)\Documents\My Games\KillingFloor2\KFGame\Config
In this folder find the file named KFInput. This is an ini file or rather a text file. Open it up and find
[KFGame.KFPlayerInput]
If you scroll down you will see a list of Bindings.
Here is where you can put your Bind for example I can add Roll the dice to my H key if I paste
Bindings=(Name="H",Command="Say !rtd",Control=False,Shift=False,Alt=False,bIgnoreCtrl=False,bIgnoreShift=False,bIgnoreAlt=False) right under the other bindings. (No Spaces). You can click save to save the ini file and then open up the game and press H and it should say !rtd in chat thus rolling the dice in the server.

With every KF2 major update Tripwire has reset our binds and this can cause us to lose our keybinds. So the best way to keep this from happening is to set your 'KFInput.ini' file to Readonly (Located in C:\\\Documents\My Games\KillingFloor2\KFGame\Config) This will keep the file from being updated when KF2 updates, however this could mess the game up if Tripwire changes anything in this file so the best way may be to just back-up your file and then copy paste your special Keybinds every update.

There are many binds that can be set out there that I'm not going to list here, but binds can be very useful espically to Admins/Mods so give it a try.
I hope ya'll found this useful and sorry for the wall of text.

-Special thanks to Bryan for helping me learn how to set-binds to the numpad keys and keep binds every update!