Upon searching, most hits were ideas for new virtues or related to apprenticeships. I know it's possible to learn additional supernatural abilites in Ars (and there is a formula for it). I also know it's possible to be taught other abilities via a teacher.
My question is whether it is possible for a magi to learn additional virtues during the campaign. For instance, my magi in my current campaign has come across many instances where being able to have more combat oriented casting abilities would be extremely beneficial and I can see him studying or even asking for a teacher to help him become a better combat support mage.
I couldn't find anything in the main rulebook about a mage teaching virtues they possess to another magi outside of apprenticeship. My guess is that in many games, as well as according to book lore, magi don't part with their knowledge easily or freely which is why it might not happen?
Teaching virtues is normally not possible, and the mentions of that happening during apprenticeship are late additions needed to explain how apprentices learn.
One can gain virtues during play in other ways though.
Mystery initiations are one of the more common ways for that. (Rules in The Mysteries: Revised Edition)
Going on a pilgrimage can give you some virtues (rules in The Church)
And then there are various story events that can result in a new virtue (or flaw), but that is all case-by-case and up to each group to decide.
Of course, Faerie stories can grant virtues - see Realms of Power Faerie p26 (becoming faerie through fable) and p39(Leaving Eudokia).
Infernal pacts can grant you anything you desire, at a terrible cost.
As Visandus pointed out, if you enrich objects of virtue (Realms of Power: Magic) someone with Magic Lore can gain the appropriate virtue. There's quite a few examples given in various tribunal books.
It should be noted that Objects of Virtue, when Enriched, have to penetrate to grant the Virtue, so Magi do not gain the benefit of the Virtue, unless the object of Virtue is part of their Talisman.
It may also be worth taking a look at Houses of Hermes: Mystery Cults if some of your troupe play a Bjornaer, Criamon, Merinita, or Verditius. Those houses have specific mysteries related to their interests. The Mysteries describe more general paths. The actual initiation mechanics are essentially the same.
Until you get "The Mysteries" (and I agree with all who recommend it), in a nutshell:
Cults are secret organisations, usually based on old non-hermetic traditions, that knows techniques, called initiations that allows to grant virtue; Cults are focused on specific interests/goals/beliefs;
These virtues are tailored to the Cult interest and tradition to help his members;
A member of such Cult can request to be initiated earning specific virtue;
Initiation requires sacrifices (of time, wealth or specific items), possibly quests (going to certain places, at certain times, performing more or less risky actions, etc...);
Usually, Major virtue requires as part of the process to get a flaw (minor or major), whereas minor virtue could require only sacrifices and quests.
It is grossly simplified, as not to disclose too much of what is contain in the book, but it gives you a snapshot of how to proceed.
So, it usually means that a mage as a keen interest in a topic, let say Alchemy or Star magic, and does some research. He might found indication of such Cults, or his research attracts the attention of such Cult. If the Cult deems him worthy/interesting, they will approach him and after interview and investigation cold offer him to join the Cult.
He will slowly get acquainted to the Cult tenants (develop a new skill Cult Lore) and then could request to be initiated into their Mysteries (aka getting specific virtues).
Most Cults have a vow of secrecy, sometimes oath of fealty, so it is not "pick and choose" what you want and shop amongst various cults - although it is possible to be part of several cults as long as their tenants, vows or oath don't contradict each other.
It is probably my favorite extension to the Order of Hermes and the base rulebook.
