Adding variables to C++ control rig classes

You can declare variables in control rig classes the same way you would in any blueprint class:

UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Control Rig")
bool bUseFullBodyIK = true;

You must ensure that "Show Inherited Variables" is enabled by clicking the gear icon in the MyBlueprint tab, otherwise the variables won't show up. If you've done this and still can't see the variable, restart the editor.