I don't think admin will approve this method, but it might help.
In 4POS Touch, you have a function to create a "Quick Button" in which you can input the description and price of a product without the need to go to the Back Office.
So if you're selling a one of a kind antique and you haven't listed it yet, simply create a Quick Button for it. Just remember to delete the button afterward in 4POS Touch, and disable and discontinue it in the Back Office.
This isn't the right way, but it's a way - i think