You could renumber an unused adjunct to 5803, then put a primary coverage button for 5803 on the phones that should answer it (up to 8 extensions).
To find an unused adjunct: First find out if you have any MLX or TDL cards in the system, and which slots they are in. Do this in WinSPM, in the Standard SPM screen, then select Maintaince > System > Inventory. This will let you step through the cards in the system, and which slots they are in.
Once you find the first MLX or TDL card, re-enter maintaince, go to Port > Station, then dial * and 2 digits for the slot number, and 01 > Enter > Status > Next
You are now looking at the first adjunct associated with the first station. Look at the extension number. Is it part of your current dial plan? If so, press Next twice and look at the next one. Once you find an unused adjunct extension number(probably in the 7300 range)write it down, and renumber it to 5803.
System Program > Single > Adjunct > Old Adjunct Number > Enter > 5803 > Enter
Go into Centralized Telephone Programming or use the Quick Access for phone programming for the phones that should answer 5803. Select an unused button with a light, and set it for Primary Coverage 5803 with Immediate Ring.