I´m getting late here, and I wanted to add something.
Per kodak profesional guide you will get a combined focal lenght when using positive or negative supplemantary lenses. The formula was alredy mentioned and is combined focal length = 1000/power of the lens in diopters + the diopters of the supplementary lens.
The required exposure when combination is focused at infinity or at normal distances is Efective f number=indicated f number x (combined focal length/camera lens focal lenght). Of course you need to calculate aperture for combined lens once and the use the same correction if you are working at normal distances. For close up work you need to calculate exposure using the combined focal length and the bellows extension. If you leave the camera lens focused to infinity and then add the supplementary lens and instead of refocusing you move the whole camera to focus the subject, then i believe it is correct to think you dont need any exposure compensation. But at infinity you will get a faster lens with the combination of a lens + a positive supplementary lens and slower lens with the addition of a negative supplementary lens.
You will get better quality if you put the supplementary lens on the back of the camera according to this book.
I´ve been using Nikon 5T and 6T achromats on back of my 210 mm lens with a very good quality. i get combined focal length of 160 mm and 130 mm. If I measure film distance to center of lens this measure longer than calculated combined focal length but if I put supplementary lens on the front of the lens it measure calculated focal length or little less, dont remember very well, but field of view is the same. They cover 4x5 with movements since I use a rodenstock apo Sironar S 210 mm and the angle of coverage keeps the same if there is no physical obstruction.
I also use -2 Kodak Telek suplementary lens. This is the opposite that with positive lens. This work better on the front of the lens and in this way it measures the same than calculated focal length or very close to it. By putting it on the back the distance from the film plane to the center of lens is less and in this case I get a different field of view. Well I hope this helps