I would like to add my (unsupported) solar panel data into ‘Home Energy Management’ in HASSIO but unfortuantly the only way solar is supported is using a ‘clamp’ or a energy device which is supported as ‘energy integrations’.
My solar panel data (brand is Afore) comes from a JSON call using ‘platform: rest’. The entitites I get this way are not available in the selection ‘Home Energy - Solar’.
How can I make them available / use them to provide my solar data to Home Assistant Energy?
You don’t need a full integration. The entities just need to be configured with an appropriate device class and a state class in order to be selectable for energy monitoring. See Sensor Entity | Home Assistant Developer Docs for the various options.