Merchant containers are in Storage Area. These containers seems to support stacking items also if the item normally is not stackable.

For example, take this entry from a Mage merchant, for the Heating Stand, which isn't stackable:

...
Item
{
ObjType 0x1849
Graphic 0x1849
...
Amount 20
...
}

But, if I try to AddAmount() to it, I get the usually error "That item type is not stackable".
If I try to CreateItemInContainer(), a new item stack is created, but not added to the already existing stack.

I would like to be able to add items to the merchant stacks, as items in containers in storage areas seems to support this. Some way to do this exists?

Yes Austin, but,the problem isn't that I can't create a stack of item in Merchant inventory.
The problem is that, when using that function, items created are not added to an existing stack in the Merchant inventory. They are created as a standalone item.
I've tried using also CreateItemInContainer(), no luck.

PS: I'm on POL096

Ah gotcha..
Maybe AddAmount() in UO.em needs an override.