Walliro team try to integrate as many Walliro Apps as possible into Walliro. But you can also add Walliro Apps that are not listed by default in the Apps section as long as you have the URL for a specific Walliro App.
To add custom Walliro Apps, just go to Apps > Add Custom App. You will then just have to paste the Walliro Apps link into the input field and click on Add. Apps need to integrate with the Walliro Apps SDK in order for them to be compatible with Walliro. Make sure you trust the developer of the Walliro Apps, as using custom apps is at your own risk.