I attempted this while trying to use coriolis in a shadowrun style game..
Not easy, we came up with make a skill in each circle of magic,
ie. Circle of fire, circle of life, circle of mind etc et c.
Then came up with a bunch of spells and gave them prerequisites.
Fire ball spell - required Circle of fire 2
Flame barrier spell - required Circle of fire 2, circle of warding 1
Frenzy spell - required circle of mind 1, circle of fire 1
A character had to have the skill ranks in all the skills to use the spell. then make a normal skill roll to use it.