You'll need both GURPS 4E and GURPS Voodoo (which apparently hasn't been released in PDF in the "GURPS Classic" line yet) to make use of this material, as all of the background and non-game mechanical stuff is in there. I have not explicitly included what spirits can do yet. I am working on a spirit template for GURPS Voodoo games, but it's the most complicated part of the whole conversion.
For all Possession effects, see GURPS Voodoo for special effects that apply. For Spirit Warrior templates, use the value given as the base for the Shapeshifting (Alternate Form) advantage. Note that if a character has some of the advantages and disadvantages involved in the Possession effects, it will change the Spirit Warrior base value.
The stats are long, so they are behind a cut.