It's possible, but difficult. I don't have time to detail it all right now but in summation you have to have all DH options fatigued below their auto rest. Sparky will then put a pitcher in at DH. However, if you've only got one pitcher that can hit, there's no guarantee that Sparky will use him. Sparky seem to choose the pitcher for DH based on which pitcher has he most RL ABs or something else totally unrelated to hitting ability.