Yes, you can add additional steps to any wizard defined in web-client-config-wizards.xml.
You have copy the whole configuration for the wizard you want to change and copy it to your web-client-config-custom.xml file. You are then free to add extra steps to the definition. When the app then requests the "createWebsite" wizard yours should get picked up.
However, in this scenario there is a potential upgrade issue in that you have to copy the whole definition. All you'll need to do though is copy the definition from the new version and re-apply your extra step (obviously that is only true if the wizard definition changes from one release to the next).
Hope that helps.