THIS IS A WORK IN PROGRESS!
Some ideas were contributed by Kelly Pedersen.
A magician needs to buy three separate abilities, in GURPS terms: Magery, Summoning, and access to the Astral Plane.
Where a spell would normally use the -1/hex modifiers, it instead uses the Range/Distance table [+50%], but this is Sense-Based (Vision) [-20%]. Magery is therefore subject to a +30% overall cost modifier.
A character with access to Blood Magic may inflict damage on himself or others (with an appropriate ritual implement) and use this to power his spells: each hit point of damage gives 2 fatigue, and this damage may be magically healed. This knowledge is considered a Perk.
Modular Abilities: Spirit Summoning (4 points/slot + 4 points/point of ability; Social [+0%], Requires Will Roll [-5%], Limited: Only Allies with Minion and Summonable [-50%], Magical [-10%]). Typically there will be ten slots for a total of 44 points.
This 135-point package consists of two powers:
Jumper (World) : Interplanar Only [+0%]; Limited Access (Astral Only) [-5%]; Magical [-10%]; Projection (creates a physical body on the destination plane) [-25%]; Reduced Time 2 [+40%].
Clairsentience : Aware [+50%]; Clairvoyance [-10%]; Limited Access (can only see current location on Astral) [-50%]; Magical [-10%]; Nuisance Effect (visible and vulnerable to Astral entities) [-10%].
Shadowrun spells are learned individually in the same manner as GURPS spells. They have no prerequisites.
The maximum Force value usable by a caster is twice his Magery attribute.
The fatigue cost of casting a spell equals its Drain Value, minimum 1. This is affected normally by the spellcasting roll.
Area spells have a (Force)-yard radius. Adjusting this upwards increases the spell's fatigue cost (multiply by new radius, divide by original radius). There is no fatigue cost reduction for decreasing the radius. Each yard of radius increase or decrease incurs a penalty of 1 to the casting roll.
Note that ranged spells do not need a separate roll to hit: if they are cast successfully, they will hit their target. (The target still gets any applicable defences, of course.)
Spell damage can be converted normally: multiply the damage value (i.e. the spell's Force) by 4/7 to get an approximate number of dice. Apply the stage 3 procedure from Adjusting Damage (GURPS High-Tech, page 166) to convert this to dice+modifiers. Or use this table:
Damage types should be obvious: fire spells do burn damage, etc.
Most other aspects of spellcasting (ritual magic, etc.) work as in GURPS.
A magician's tradition will include a table mapping spell categories to spirit categories. Only the appropriate type of spirit can lend assistance in spell use.
The spirit gives a bonus to casting a single spell equal to its Force rating.
Initiation allows the character to buy higher levels of Magery. Of itself this has no point cost, but it will generally be bundled with Rank (Initiated), which will cost the usual 5 points per level.