A short-cut may be the general minor virtue "Latent Magical Ability", which I have always interpreted as meaning that a character has an inherent propensity or potential to spontaneously learn an un-anticipated supernatural (magical) ability. If the right circumstances occur, and whatever the right circumstances are, your character has not yet been exposed to them.
Exactly how big the Supernatural ability is, is poorly defined. LMA certainly can be swapped out for a minor supernatural virtue, and I have seen suggestions that it can be swapped out for a Major Supernatural Virtue. If the character is a Magus, then Hermetic virtues are apparently included under "supernatural virtue", which makes it sound like some mage could spontaneously develop the Gentle Gift after character creation.
But there might be a hint that Latent Magical Ability might give you access to more than a single supernatural virtue. I seem to recall that I read somewhere that someone's character managed to learn a second magical Tradition through the intervention of LMA. Definitely something your troupe would have to agree on.
Then there is the case of Bjornaer. If I recall correctly, her origin story was that she was initiated into an infernal shapeshifter tradition, but when she communicated with an ancestor spirit she learned a whole new magical tradition based around the HeartBeast virtue. Nothing is explicitly said that she had a Latent Magical Ability, so I am merely speculating. I think I am going off on a tangent from the original post, and more towards what makes a magician be the source of a Mythic Bloodline.