I wouldn't give them turning, even with a feat...
But maybe with a spell... Several spells, actually. Level 3, 6, and 9.
vade retro, lesser 3rd level: Swift action spell that lets them turn and destroy undead as a cleric of their caster level -4 (maximum 10, reached at level 14).
vade retro 6th level: Swift action spell that lets them turn and destroy undead as a cleric of their caster level -2 (maximum 15, reached at level 17).
vade retro, greater 9th level: Swift action spell that lets them turn and destroy undead as a cleric of their caster level (maximum 20, reached at level 20).
What do you think?