A lot of necromancers seem to live a very long time without going full undead, for a given value of living. It's extremely unlikely any of the underlying theory could be used without heavy dhar nonsense however.
For somewhat resisting old age, normal healing magic can probably make you last a bit, and we got a hint from the Empress about schmoozing up to Shallyans can also help a lot, but those alone would probably only serve to buy you a few decades, not make you immortal. If that can be done at all without evil bullshit it's probably either via direct divine intervention, or maybe with Qhaysh, if anyone would have ever researched that. Assuming we need it somehow instead of just slowly becoming an Ulgu elemental, our best bet is probably to build a massive amount of divine favor, though I'm not sure how much Ranald would like that plan unless you make it exceedingly funny somehow.