I'm pretty sure I've tried this before and it's not possible. Because the step stool can't have items placed on top of it, and the NPC merchants are basically just another piece of furniture...

However, you might be able to use the new partition + wall shelf method to place the merchant at just the right height to look like they're standing on the step stool... But I'm not sure if this can only be used for "tabletop" items...

I hope it works! I mean, what's the point of a Lalafellin step stool if you can't have a Lalafel standing on it? XD