There's no way to add items in the Trainer yet.
However, youc an do a very easy trick to accomplish that.
For Armor items, I suggest you freeze the rupee count with the trainer and buy new armor from Hateno or Kakariko, then replace that new armor with the one you desire (Sacrificial Armor).
For items in general, same thing, although for this I suggest you use a very mundane and common item such as the Apple, you can farm those in Hateno village, grab one, change it to a new item, grab another, change it to another, etc. Repeat.