If users type in their (premium) credentials it is legal. But I still cannot just take someone else project and put my things on top if it ain't open source an

If users type in their (premium) credentials it is legal. But I still cannot just take someone else project and put my things on top if it ain't open source and it isn't and there seems no indication of wanting collaboration. So I could contact him if I ever made anything, but I cannot just make and release it.