Which is why in modern times, the Lineage of Pralix is the only group pretty much bringing hedge mages in outside of outlier cases. Does it happen that those not of the Lineage sometimes bring in a hedge mage? Of course it does, there are always exceptional or unique circumstances which might require it.
Find a hedge mage who has a power that is critical in some way to a goal? There a hedge mage who can provide something important to the Covenant or Tribunal? Doing research on integrating things into Hermetic magic theory and a hedge mage has any ability that if integrated will increase the power of all Magi who learn it? Discovered a "hedge mage" who is actually closer in power to a member of the Order or even a founder? If they are at all compatible with your House, snatch them up. If they are not, look for someone in a compatible House (most likely Ex Misc) to do it.
Granted all those are at the edge of possibility. Most cases that do not involve the Lineage will be some hedge mage who did something good for the [Magus, Magi, Covenant, Tribunal], they are Gifted, and one of those people decides to sponsor them/find a sponsor for them. You still need a compatible House, but depending on who they helped they can easily have enough support to overcome political roadblocks.