Don't recommend it. A 28-200mm is very likely to be optically not very good. Adding a 1.4x or 2x you would be just magnifying the defects of your lens. And you would very likely loss autofocus as well, as autofocus normally stops functioning around f8 due to loss of light and decreased contrast at f8. However, no one's going to stop you if you really want to, and happen to be satisfied with the end result.