You need to finish quest given in battle academy, for every class different but here it is :
mage you need to kill 20 mage units (archmage, necromancer,demonolog. . . )
paladin 20 holy warriors (priest, inquisitor, paladin)
warrior 20 fighters (knight, horseman,assassin??? wtf why assassin i have on idea, but there he is). After you finish quest you need to go back to debir to get reward in battle academy and youll get new skill available in your skill tree.
As for not being able to use `old` playthrough it`s impossible to copy save ifles form 1 mod to other. Every new mod to game need new game started so tough luck with that. If you`r interested here is nice savegame i got on this forum (i think it`s bladeking`s but i`m not sure it`s early save mage impossible) , i wont spoil game but in the end i managed 91 int build, and all dragon items are available (hearth, drakonix). So you can go draogn support or high spell dmg.
So have fun
