You can sell items from the Catalog in POS as a special order.
In POS, type the SKU and press Catalog (Ctrl+K). If the item is in the Catalog, the Catalog will display it on the Item Detail screen.
On the toolbar, press Sell (F11).
From the Catalog's Item List Screen, select the line item you want to sell in POS.
On the toolbar, press Sell (F11).
From the Catalog's Item Detail Screen, press Sell (F11) on the toolbar.