Stalin was a Marxist-Leninist. This is very clear from all of his writings and his economic policies.

He wrote this text, detailing his interpretation of Lenin's additions to Marxism.

Yes, he did. Stalin was the one who codified the idea of Marxism-Leninism, which is the ideology that most Communist Parties in the world subscribe to (or at least have their own ideology based off of). Lenin referred to himself as a Bolshevik, but Stalin referred to himself as a Marxist-Leninist, believing that Lenin had contributed significantly enough to Marxism that it was now an entirely new stage (and to be honest, it's kinda hard to disagree - Marx provided the theory, but Lenin and the Bolsheviks were the first to put it into practice on a state-power basis). In his 1924 work The Foundations of Leninism, Stalin says:

“Leninism is Marxism of the era of imperialism and the proletarian revolution. To be more exact, Leninism is the theory and tactics of the proletarian revolution in general, the theory and tactics of the dictatorship of the proletariat in particular.”

And in his 1926 work Concerning Questions of Leninism, Stalin references this exact quote of his, and elaborates thusly:

Is this definition correct? I think it is correct. It is correct, firstly, because it correctly indicates the historical roots of Leninism, characterising it as Marxism of the era of imperialism, as against certain critics of Lenin who wrongly think that Leninism originated after the imperialist war. It is correct, secondly, because it correctly notes the international character of Leninism, as against Social-Democracy, which considers that Leninism is applicable only to Russian national conditions. It is correct, thirdly, because it correctly notes the organic connection between Leninism and the teachings of Marx, characterising Leninism as Marxism of the era of imperialism, as against certain critics of Leninism who consider it not a further development of Marxism, but merely the restoration of Marxism and its application to Russian conditions.

As for if Stalin had "his own" in-depth ideas about Marxism, it's kind of yes, kind of no. He certainly expanded upon Lenin, and many of the things he did as leader of the Soviet Union he extrapolated from Lenin's writings or ideas; there's very little that Stalin actually did on his own in terms of theoretical additions to Marxism. If anything, Mao would be one who added heavily onto Marxism, as he formulated the concepts of the Mass Line, Protracted Peoples' War, New Democracy, organizing the masses to fight against revisionism within the party (i.e. Cultural Revolution), and much more.