Here's your answer. If your pump is not damaged this will work.
Take the pump outlet fittings off Take a shop rag and an air gun. Put the air gun in the fill port, and wrap the shop rag around it real tight to make a seal. Blow all the air you can into the case until oil comes out the pump fittings. Have an assistant immediately reassemble the fittings on the pump while oil is still leaking out. This pushes oil into the pump. immediately start the tractor when the fittings are tight. You can safely pressurize the tank to 5 PSI, but don't make a contest out of it. You need clean dry air. And in the future when you drop oil, add it back right away as soon as you can. This can happen easily with worn pumps, or if the sump is left dry for a long time. If you look in the port, and you see aluminum filings DONT START IT. the pump is trashed, and you will just ruin your steering and rockshaft valve if you try to use it. Get a new one - the old one will never work if there's trash in it from running it dry.