The ifit is actually a barrel kit in and of itself- not just an adapter, so you would need to buy a set of ifit inserts to use with the back.
Also, using the Phantom version of the ifit also requires a larger replacement pump handle.
But yes on a pump, using a barrel adapter largely negates the biggest advantages of using a barrel kit with a pump.
I don't know of a cost-effective way to use a cocker-threaded barrel kit on a Phantom.
I know of some expensive options, though- like replacing the body tube.