Adding additional spacers may require longer screws for the butt pad to hold all the spacers. Longer screws can be purchased at your local hardware store. The thread pitch is 1/4-20.
Depending on where your stock was cut for the spacer system, HTG and A3 spacers may not be a perfect fit. Some grinding to fit may be required.
Spacers will vary in shape from the picture to fit the correct spacer system.