What about this House-Rule:
To increase the force of a spirit, the mage rolls his/her magic vs the spirit's force. Every 2 net hits will increase the force of the spirit by 1 point. The ritual requires a magical lodge, ritual materials, and a number of hours equal to the difference between the spirit's current force and desired force. If the mage fails, the materials are expended and the spirit's force does not increase. On a glitch, the mage may lose 1 or more points of magic for and extended period of time. On a critical glitch, the mage loses 1 point of magic permanently and the spirit's force is reduced by 1.