Based on Source SDK's document

Adding a new player class to the Source SDK requires a mix of C++ and editing keyvalue "scripts"

It's actually a fairly simple task.

Just keep in mind that each gamerule determines its own classes.


For example if you want to edit the classes for CClassicGameRules:

1. ClassicGameRules.cpp

	You will see g_ClassicGameRules_Classes_Blue and g_ClassicGameRules_Classes_Red and a constant CLASSICGAMERULES_CLASSCOUNT.
	You must have the same number of classes for each team for now.
	
	The two arrays serve as internals identifiers and identifiers for which playerclass scripts to load in the "luminousforts/scripts" directory.
	
2. Create a playerclass_blue_class*.txt and playerclass_red_class*.txt script for your class using another playerclass file as your template.
3.
	
Step 1: Open sdk_shareddefs.h and sdk_shareddefs.cpp
	(.h) increment the define SDK_NUM_PLAYERCLASSES to reflect the new count of player classes you will have

	(.cpp) modify pszPossiblePlayerModels, and add any player models you'll be using.

	if You're using teams; 
	(.cpp) modify both pszTeamBlueClasses, and pszRedBlueClasses to add your new player class.
	if you're not using teams:
	(.cpp) modify pszPlayerClasses and add your new player class.


Step 3: create new .res file's for your new classes inside the "classes" folder - This is for the player class screen, where it shows the information for the class.
Step 4: modify resource\ui\classmenu_XXX.res to add new buttons to select your new classes.
Step 5: modify your language.txt file to add any new localization strings (ie: descriptions) you're using.
Step 6: Compile your mod, load it up, and if everything was successful you should have a new class to choose!