They're essentially a distinct subset of the rules, optional to the main rules. You'll have to adjust the cost of a few things like Characteristics, and add in CV/MCV which used to be figured from DEX and EGO, respectively. Otherwise I don't think they'd be problematic.
Those rules are fully laid out in two supplementary source books for Fourth Edition HERO System: Hero System Almanac I, and Horror Hero: Endless Nightmares. Both are available in this website's online store, at very reasonable prices.
Our sometime forum colleague and Hero Games author, Michael "Susano" Surbrook, hosts a full 4E write-up for the Lord of the Nazgul using the Spirit Rules, on his tremendous HERO System website, Surbrook's Stuff.
