You can activate KNX scenes with Thinka. You configure the scene group address and scene number. Optionally you can configure a second scene number for the Off scene.
This scene is available in Alexa as a Scene, and in HomeKit as a Switch. In HomeKit the switch will trigger the On scene when switched on. Likewise the (optional) Off scene will be triggered when switching off.
You can extend existing KNX scenes with HomeKit actions by adding automations to the switch in the Apple Home app. Open the Home App and select the Automation tab. Create a new automation that is activated when the scene switch turns on. Then you can select the scenes and accessories to automate. Once you've saved the automation it will be triggered whenever you activate the scene from Siri, the Home app, or from KNX.