In killingfloor/system folder, there should be a ServerPerks.ini file. Edit it with your favorite text editor.

To add custom weapons to the trader, follow suit with the rest of the Trader Inventory.

TraderInventory=#:Package.WeaponPickupLine

[# will determine under which perk to list the weapon in]

To find the weapon pickup line, I usually look in the weapon's .upl file and copy what's after "PickupClassName=".

Note: this is how I've always done it but it hasn't worked for me since the recent update. I feel I should reinstall KF just to make sure it isn't because of all my modding that messed up the update.

Also note: having serverperks and weapon shop mutator running at the same time will most likely cause conflicts as they both modify the trader menu.

Cheers

@Kuebic is there a video tutorial available for everything you just said?

Not that I'm aware of... it's a bit more intuitive than you think. I might make a video later, but essentially find your serverperk.ini file and modify it to your liking.

To look back and see how difficult this all was at the time compered to how simple it all seems now.
...
Only question now is...> Did you make the video<......
:3