Configuring overrides

I need some help configuring the overrides in our MAS 500 system. Our GL Acct #'s are formatted with a 5 digit natural acct, then a 2 digit location code, then a 2 digit customer code, then a 2 digit product code. We currently have 3 locations (warehouses) active. When I invoice regular stock shipments they post properly to the sales and COGS GL#'s based on the overrides (i.e. 31000-10-00-05). However, when I create a drop ship sales order for an item that we also stock, it forces me to select a warehouse location, which I think defaults from the customer's information, and then when the drop ship is invoiced, the sales and COGS GL#'s that get used are for that warehouse location. I have set up a separate GL segment (99) that I want to use for drop ships. When I post a drop ship sale, I want it to recognize it as a drop shipment and post under 31000-99-00-05. Is there a way to configure our system so that it will do this without actually setting up a "drop ship" warehouse?