If a spirit has the Endowment power, can they share their Immunity to Age/Normal Weapons? Excluding Hidden Life/Spirit Pact, are there any other ways for characters to gain the benefits of these abilities?
A spirit can't Endow a power it doesn't actually have. No spirits have Immunity to Age and no spirit any sane metahuman would want to be involved with has Immunity to Normal Weapons.
- Materialization spirits only receive it when materialized; they don't have it naturally.
- Possession spirits don't grant it via any means.
- Inhabitation grants it via Hybrid Form or Flesh Form merges... and no rational person wants to become a filthy bug spirit. True Form merges become Materialization spirits.
The only spirit that actually gets Endowment is a Great Form. In theory, a Great Hybrid Form Insect Spirit or a Great Flesh Form Insect Spirit could Endow somebody with Immunity to Normal Weapons, but you wouldn't want to be beholden to such an entity, nor the Insect Shaman or Queen that controls it. Besides, it'd make you harder to eat.
A character could get Immunity to Age by being exposed to HMHVV-I (or Ia) and becoming one of the Infected (Run Faster, p. 133-). Banshees, Dzoo-Noo-Qua, Goblins, Mutaqua, Nosferatu, Vampires, and Wendigo all get it. The HMHVV-II species and Ghouls (HHMVV-III) don't grant it.