In GURPS Voodoo all spirits have an ability to alter probability. Ignore it. It occurs in no traditional magic system and is nearly impossible to implement and preserve game balance. If you really like it I suggest dropping the mechanism and allowing spirits to buy Luck or Super Luck with the Affects Others (+40%, use Will instead of DX to hit) and Costs Fatigue (about 5 points -20%) enhancements. At minimum the Voodoo mechanics need a ruling on the minimum initial probability, I'd suggest if the initial chance is less than 3 on 3d (1 in 216) a spirit can automatically prevent it, but not encourage it to happen.
The ability to manipulate physical objects without touching them. In Voodoo all spirits have this ability, and it works well for the minor feats of telekinesis some magicians display. It is probably possible to build it from Telekinesis with limitations, but the feel is rather different. There is a substantial energy cost, 1 fatigue per 2 ST per 'action'. I would be a bit more generous than Voodoo on this, an 'action' must be a single feat, but could last several seconds - long enough to move an object entirely across the table or write a complete word. Range is no more than 3 hexes. If any sort of precision or skill is called for there is an added cost of 1 fatigue for a DX of 10, plus 1 fatigue per 2 additional DX. A direct attack requires a DX roll and inflicts Thrust -3 damage based on the ST provided. GURPS ST is a bit erratically defined, but depending on your choice of scaling law ST 2 is about that of a 20-40 lb animal. If you can imagine a housepet being strong enough to do something ST2 is sufficient.
Option: Allow magicians to learn DX skills usable only through this ability. Sure he could spend fatigue for a higher DX and operate at the default, but specialized skills go a long way to distinguish this from psionics, and allow an interesting transitional step to the GURPS Magic spell system.
Almost certainly the most common such skill in modern fantasy. An ordinary lock can be opened using ST 2.
Uses the normal Parry Missiles rules on CI p135. It can deflect a missile weighing up to 6 x ST lbs.
Allows a single hand weapon parry at 2/3 skill. Minimum ST is 6, which makes it too expensive for sustained use, but sometimes once is enough.
The ability to translate objects into the Spirit World. This costs 1 fatigue per pound per minute or fraction thereof. An object brought into the spirit world can be manipulated by spirits and becomes intangible on the physical, though a magician could move it with Poltergeist.. Physical barriers may be circumvented by this effect - a spirit with can appear to teleport objects a short distance by dematerializing them and carrying them to the new location. Objects don't retain motion between the worlds, so a spirit can't fire a dematerialized gun and rematerialize the bullet (or it could, but the bullet drops harmlessly to the ground). An object can't translate into anything 'solid' in either world. If you place a dematerialized object somewhere it must to return to the material world the effect can't end. It continues to draw fatigue every minute (until you are destroyed, then it remains until something moves it or the obstacle, when it snaps back.)