Starting new magic paths during the game

[Skirmisher]

Is there any way for a commander/pretender/prophet to learn a new magic path (type) after the game starts?

And besides empowerment, is there another way to raise your magic levels in paths you already know?

[Micah]

Empowering will add a new path, as well as propheting a unit (they get H3 from it)

Twiceborn sometimes gives a D bonus when the mage dies, and I believe being lost in time and space can increase the astral level of the victim. Neither is a particularly reliable method though, especially with all of the D boosters available instead of messing with Twiceborn. It's pretty hard to get lost in time and space to begin with, and usually comes with some insanity, if you even get the unit back at all.

Wishing for magic power also provides a path boost.

[MaxWilson]

Micah's answer is thorough. The most common way to raise your magic paths, Skirmisher, is to use booster items like those listed at the back of the manual. It's cheaper than empowerment. Experience does NOT raise the paths on units although it does give them better precision--it's not like D&D where you get more powerful spells as an individual gains experience. The fact that Sauromancers can cast Drain Life and Augur Elders can't (without booster items) is an intrinsic feature of both unit types.

-Max