You cannot. Magical power is all tied up with free will, you trade your free will for magical power. Gifts and geases are one of the few places where this free will loss actually manifests in game terms, Sandy Petersen's sorcery rules is another, but it is pervasive in Gloranthan magic. If you take away someone's free will, they do not have access to it in order to initiate a magical transaction.
There was a set of heroquesting rules knocking around a few decades ago in which WIL was a stat. You build it up in normal play and then start spending it on heroquest powers, or something like that.