Maybe through some kind of magical encounter. For instance, he could stumble upon a portal that leads to a world where the Keyblade exists and be chosen by it. It could be that his strong will and desire to protect his friends are what draw the Keyblade to him.
In fanfiction, Naruto could obtain the Keyblade if he was in a situation where he had to prove himself worthy. He might have to go through a series of trials or battles, showing his courage, loyalty, and strength. His ninja training and experiences would give him an edge in these challenges. Once he passed all the tests, the Keyblade would choose him as its wielder.