Find the CoreAbilities.xml file and search for consumespirit.  Edit the ability to match the following and it will appear in the Mage tree.  I would prefer to make it a spell you get from a quest, but haven't figured that out yet.

  <AbilityBonus InternalName="Spell_ConsumeSpiritAbility">
    <AbilityBonusType>Unit_Level</AbilityBonusType>
    <AbilityBonusType>Unit_Spell_Rank</AbilityBonusType>
    <AbilityBonusOption InternalName="Spell_ConsumeSpirit">
      <DisplayName>TXT_ABILITIES_SPELL_CONSUMESPIRIT_DISPLAYNAME</DisplayName>
      <Description>TXT_ABILITIES_SPELL_CONSUMESPIRIT_DESCRIPTION</Description>
      <Icon>T_ConsumeSpirit_Icon.dds</Icon>
      <PerkTreePosX>5</PerkTreePosX>
      <PerkTreePosY>1</PerkTreePosY>
      <GameModifier>
        <ModType>Unit</ModType>
        <Provides>TXT_ABILITIES_SPELL_CONSUMESPIRIT_PROVIDES_UNIT</Provides>
      </GameModifier>
      <CombatRating>20</CombatRating>
      <IsCombatAbility>1</IsCombatAbility>
      <Prereq>
        <Type>AbilityBonusOption</Type>
        <Target>Unit</Target>
        <Attribute>Affinity</Attribute>
      </Prereq>
      <Type>Spell</Type>
      <AIData AIPersonality="AI_General">
        <AIPriority>5</AIPriority>
      </AIData>
      <Cost>1</Cost>
    </AbilityBonusOption>
  </AbilityBonus>