How about this?
Memory Infusion(Su): When the Lich slays an opponent using the Draining Touch, the soul and memories of the opponent doesn't merely vanish. Instead, the Lich is able of capturing the spirit of his departing foe, turning it into necromantic energy which can be stored and used by the Lich.
When a Lich slays an opponent by using the Draining Touch, he automatically absorbs the memories & energies of his opponent. This has two effects. First, the Lich instantly gains access to all memories of his slain opponent, as with the Dopplegangers special ability. Second, the Lich stores the life force of his target. For each HD of the target, the Lich gains 100 xp. which he can use either to create Necromantic items or cast Necromancy spells. These experience points can be stored for as much as one month. If they are not used either to create a Necromantic item or to cast a Necromantic spell, they are lost at the end of the month.