I hope your talking about the Death Knight spell "Path of Frost", simply add this into your FightClass:
<FightClassSpell>
<FightClassConditions>
<FightClassCondition>
<ContionType>Buff</ContionType>
<Param xsi:type="FightClassConditionStringBool">
<Name>Path of Frost</Name>
</Param>
</FightClassCondition>
</FightClassConditions>
<SpellName>Path of Frost</SpellName>
<Priority>30</Priority>
<IsBuff>true</IsBuff>
<CombatOnly>false</CombatOnly>
<LockFrame>false</LockFrame>
</FightClassSpell>