Unfortunately, apps cannot be added manually because apps need to be integrated within drupe by our developers.
Here's an analogy that may better explain the situation: there are different standard electrical outlets around the world, hence some electrical appliances require adapters in order for their plugs to work with outlets they don't match. Think of drupe as the outlet, and of the different apps as the different plugs. We need to create the adapter for each app in order for it to work with drupe.
Nonetheless, we're constantly adding more and more apps for drupe to feature, so stay tuned for more!