Currently, Spine does not support adding custom SVG bone icons directly. The ability to specify bone icons in JSON data is not available, though there is a plan to include this feature in the future when exporting non-essential data. For now, you can only use the existing set of bone icons provided by Spine. If you have specific requests for new icons, you can suggest them, and they might be considered for future updates.

Just as Spinebot said, you can suggest new ones, but we have to manually add them in. In the most recent version there are even more!