It is possible to use custom weapons in your levels for Synergy. Take note though, this feature is for advanced users only.
The first thing that needs to be done is creating your weapon script file.
You can create your weapon script file by copying an existing one or writing one up from scratch. Anything you see in an existing script file is customizable.
You should save your weapon script file in the "scripts/" directory. Give it a unique name (something like "weapon_testweapon.txt"), for this name will be referenced in Valve Hammer Editor.
If your weapon requires a new sound script file, you can create one and place it in the "scripts/sounds/weapons/" directory.
In Valve Hammer Editor for the weapon's Properties, you use: