Liches do not require access to the school of Necromancy; the only requirement to become a lich is to have a phylactery, and that only requires Craft Wondrous Item, the ability to cast spells (any kind of spells), and caster level 11th (and a lot of money and XP).
Liches also do not have special affinity for necromancy; their magical talents apply equally-well to all schools of magic.
In fact, the word “necromancy” does not appear anywhere in the Lich monster entry on the SRD.
But anyway, getting spells from a banned school as a wizard can be done. A wizard cannot use items of banned spells without Use Magic Device, but he can use UMD. Eternal wands from Magic Item Compendium don’t even require that a spell is in your spell list, so those shouldn’t even require UMD. Banning schools as a wizard also doesn’t affect other classes, so multiclassing is an option. But neither of these is casting the spell as a wizard.
For that, options become somewhat more obscure:
Recaster (Races of Eberron) and wyrm wizard (Dragon Magic) are prestige classes that let you learn spells from other spell lists entirely. No restriction based on banned schools is mentioned.
- Rainbow servant (Complete Divine) goes further, letting you learn all cleric spells, again without reference to being limited by school banning.
The Spell Reprieve feat (Lost Empress of Faerûn) lets you learn one spell from a banned school.
- The Arcane Transfiguration feat from the same source requires Spell Reprieve and Item Reprieve, so it's extremely expensive, but it does allow you to open up an entire school you had previously banned.
The Diversified Casting feat (Dragon vol. 325) lets you cast three spells from a banned school.