You know how it is when it's not till later that you catch the pun--not the one you didn't get but the one you unintentionally made!
So here's how it comes together: an INSOLE, noun, is the inside of a shoe, sometimes an extra, removable part put in for comfort. To INSOUL something...a variation of the verb ensoul...is to place, receive, or cherish in the soul or to endow with a soul.
In other words, to be IN SOUL might be a way of endowing one's self with an "insole" for the soul.
Or as Dr. Scholl would say, "Are you gellin?"