Your ballplayer can play any position in DD that his archetype supports, though most will be shown as a "secondary" position. And it really doesn't matter what position you are playing in DD as long as it's one that earns archetype points and thus progresses your archetype.
So, my base-stealing CF when I play DD is actually playing 2B. Because I have Jr. in CF and studs most other spots, 2B was the weakest spot. As long as it's just secondary, and not out-of-position, you should be okay. Except at 1B. For some reason, a guy playing 1B as a secondary seems to have a glove made of stone and balls just bounce out of his glove. Lots of baserunners as a result. So I don't recommend putting your guy at 1B even if it is a "secondary" and not "out of position". Also, I recommend putting your player in the first spot of the batting order to make sure you get maximum plate appearances in DD mode.
Hope this helps.