How can I let a pitcher bat in a DH league?

The only way I know to do this is to play players fatigued and watch them get hurt. If you set your bench hitters to rest the SIM will have to bring in a pitcher. It won't bring in your active game pitcher, though.

(If injuries are off, I don't think it's possible to make it happen: though an ejection would achieve a similar result, you can't set up for that like you sort of can with an injury.)

Someone in a progressive league with a DH did it by accident with Mike Hampton, he had Hampton playing the outfield.

I don't know what his settings were, but my guess is there were no OFs available in the player hierarchies because guys were fatigued, so the SIM picked the best option available, which happened to be great-hitting pitcher Mike Hampton. So you have to set up your rest hierarchies such that the SIM runs out of position player options and needs to turn to pitchers, but it will probably only work if the pitcher is a better hitter than anyone else available.