You can reforge and reobtain.
First method that works is using the Deeds KI thing.
You can obtain new THF pieces even after having reforged them to +3.
It's the system I used.
But I think if you store your AF on the slips, you can go to the AF reset NPC and reset it and then do it again? Haven't tried it but it might work.
With that said, despite me having a Steal focused set, I'm very skeptic on the benefit of that stat when it comes to increase the rate at which the Impish Box is stolen from the imps.
No real objective data, just a feeling.