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 10-24.
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.