Any suggestions on ways for PCs to get unnatural Willpower like they could in DH1 Ascension?

A bionic implant in the brain could do it. This could range from loyalty implants to implants specifically made to shut down parts of the brain when intense fear reactions begin.

A way to acquire it during roleplay could be to gift it to any player who has X amount of successful Fear Resist tests.

You could also allow certain archetypes/roles to purchase it just like they would a talent. Depending on what system you are using this could be a set price (500xp per unnatural bonus) or aptitude based treated like a talent (Tier 3; Willpower & Psyker -OR- Leadership).