Set Slot Item?

I'm still trying to figure out how to set the slot item to what we want. I already know the names of the potions that I want (specifically SP potion), but I just can't seem to put it in a slot at all.

I even tried setting the SP potion as a subweapon. Although the SP potion icon appeared as my secondary weapon, when I tried it, my real secondary weapon was used, which was a spear. In other words, it was only an icon graphical change.

Any info would be greatly appreciated, thanks

Dont think thats possible, the only thing you can do is refill your slots.