TMK there isn't a general rule to do so. But HoH:TL p.26ff Original Research, Apprentices p.40ff Teaching Hermetic Virtues in p.32ff Hermetic Apprenticeship, TMRE p.13ff The Mechanics of Initiation and RoP:M p.26 Advancement in p.20ff Rules of the Realm provide means and conditions to acquire Hermetic Virtues.
There is also the text box on page 92 of the core rules. While this doesn't come up too often if you use the expanded treatment of initiation from Mysteries Revised, it may be what you remember seeing.
The background to my request was that our troupe were discussing their character abilities, and two of them surprised me by saying that their Magi had the Good Teacher virtue, and since my magus character had the Apt Student virtue, it seemed worthwhile looking up that learning Hermetic Virtues rule.
I would have still needed to find out their COM ability score and Teaching skill score, but it was beginning to look like it was potentially possible that one of them could have taught my character Flexible Formulaic Magic (to go with my Flawless Magic). Already having a minor magical focus, I can't get the other's Major Magical Focus. But if I had gotten the Major Virtue, it might have been possible to learn one Minor Hermetic virtue after that, without also getting a Flaw. After all, one of them has Incompatible Arts in my favourite combination.
My character was always supposed to be able to expand his repertoire, which is why I originally invested in Latent Magical Ability. Before I was reminded of the "only 1 magical focus rule", I thought it might be Mythic Blood - my character has Puissant Perdo, strong Vim and character background said he had a striking resemblance to busts of Apromar, according to Flambeau magi.
Matt Ryan likely expected, that people reading specific rules in a subchapter Hermetic Virtues from a chapter called Hermetic Apprenticeship apply them only to hermetic apprenticeships.
Accommodating you here would require your troupe to intensely house-rule the saga.
An approach to such house rules, which doesn't affect the Order of Hermes and Mythic Europe too much, could be access to a specific part of the Magic Realms - e. g. the 'Lab of the Lost Archmage' - reserved to the characters of your troupe by some rare incident. And in that part the characters could accumulate vis and trade it (by RoP:M p.26 Advancement) for Hermetic Virtues. If it begins to get boring or sodales catch up, this access to the Magic Realm closes again.