I have a custom Application class which I want to convert to Interface and currently a lot of methods thinks that it's also Context, but when I change to Interface it's of course not, even when I implemented that interface it becomes, because implementation class extends Application. Is it possible to create an Interface which is somehow marked that is also Context? Something like expect functions.

Why would you like to do that? and no, Context is not an interface so it can't be inherit by your interface; interfaces can only inherit other interfaces

I need to merge two applications together and can't extend from two classes. I fixed with exposing getContext method and calling it when I need it.