It can't be done without a major re-write of the fatigue system.
Position players (including DH's) fatigue by PA. So if you put a PITCHER with 200 PA into your OF, he could play all season long without being tired
Pitchers SEASON fatigue is by Pitch Count... So if you put a HITTER on the mound, he would be able to pitch every game (unless tired by too many PA in games he was pitching). In game fatigue is a different matter of course, he may pitch worse during a game.
Since the different types of players fatigue by different methods, they can't overlap without a re-write of how fatigue works.