Custom State Tree State

I have a question, is it possible to make custom state tree state to add in the editor of the state tree?
I was checking out unreal engine source code trying to make it happen, and now i’m stuck not figuring out how to make it appear next to “Add sibling state”, “Add child state”:
#pragma once

include “CoreMinimal.h”
include “StateTreeEditorModule/Public/StateTreeState.h”
include “TestStateTreeState.generated.h”

UCLASS(BlueprintType, EditInlineNew, CollapseCategories)
class POSSESSIONGAME_API UTestStateTreeState : public UStateTreeState
{
GENERATED_BODY()
};

Or is it not even possible to do that?