That looks like either a spray foam roof system or an old smooth build up. The spray foam is usually coated with silicone. Unless they are recoated regularly they soak up water like a sponge. They are usually installed over old roofs and rarely installed directly over deck. Which means outside of a coating the building code will require a tear off to deck.
To add slope you use tapered insulation and have to lay it out correctly. The insulation is 4x4 pieces and you can get it in 1/8", 1/4", or 1/2" slope. It is either mechanically fastened to the roof deck or adhered. This insulation can be used with pretty much every flat roof system.
Your other options are some kind of coating but as others have said you are asking for trouble if you have water in the roof.
Adding slope is more expensive initially but you will get a longer lasting roof out of it. What you do depends on what you bought the property for and how long you intend to stay there. A tear off and new roof should get 20-25 years before you have to think about it again if done right. You could have problems our of a coating in 5 years especially since you are going over previously coated roofing with water and air trapped in it. Eventually you will have to tear it down to the deck, coatings don’t last forever. The longer you wait the more expensive it will be as inflation for construction is higher than national currently.