How to, sort of.
In merchant editor you must uncheck a box, any will do, at the end of the row. The column is "dropable" then click elsewhere within said document. Click "No" on the messagebox. then you can edit freely.
Don't use the merchant editor, though, as it overrides certain items "infinite status" with "dropable" I learned this by comparing with k-gff. There is a reference in the original of Kebla Yurt's Equipment Emporium, from Taris, where there are infinite medpacks for sale. When using kotor tool to add a shield I made, (nothing special) I found that she had only one medpack for sale from then on.