In my campaign, set in Iceland, the player Magi are inducing the local magic-users into the order. None of them are individually part of Ex-Misc, and they don't have any immediate allies of that house to draw on.
They have made them swear the oath, spent time teaching Code of Hermes, and Order of Hermes Lore, then taught them Parma.
My question is how do they kick-start them learning the Hermetic Arts? Quite a few of them (most) will have too much experience spent into their own traditions, so do they have to open the arts of very young members of the traditions, such they can be taught by both sets of masters.
Yes, you have to get their apprentices young, open their arts, and let them get taught. Within a generation or two you will end up with characters like the classic Ex Misc - hermetic Magi with a couple of non-hermetic powers and slightly unusual virtues & flaws.
The existing magi can quite happily be members of the order just using their own native magics and knowing Parma and swearing the oath. If they have too many levels of their own abilities, it may be too difficult to teach them.
Your possible options: have a min-maxed InVi lab, a lead researcher with a high Leadership and get everyone else assisting so you open their arts as a team effort (allowable by the rules as written)
Allow mystery cultists to initiate individual arts as virtues (Solomonic magi already do this, but if you allow a Hermetic Art as a virtue, there's no reason a mystery cult couldn't devise an experimental script. The reason why no-one has done this so far is that there's a perfectly good method of opening the arts that offers all the arts for no flaws).
Your magi should care about a Quaesitor in good standing witnessing the Oath of Hermes, and inform Cad Gadu of its new members. Both are mainly formalities - but help to avoid future trouble. Finding sponsors to set up Tribunal organisation in Iceland is also helpful.
There is no urgent need to teach Hermetic magic to the new Ex Misc, and not even to their pupils. But after learning the Parma, they will soon get interested. It might make sense to find one or two model students to teach first, so they can serve both as examples and as teachers for further pupils.