You can add some munich, or you can just leave it out. In a stout with that much roasted barley the difference is hardly noticable.
I've brewed 4 different British Ales in the last 3 months, all using Maris Otter. Most styles leave enough room for the the wonderful soft maltiness of the MO to come through, some, like the bitter, just a little, while others, the Irish Red, a lot. But, the dry stout had the least malt flavor and aroma from the grain - the strong flavors and aromatics from the roasted grains seem to bury the maltiness.
But if you wanted to add munich, that wouldn't do any harm - subst 1.5lb of 2 row for munich. Although the malt profile is more intense than the English malt, the difference will not be noticable against the roasted grain. Substuting munich will reduce the overall diastatic power of the mash, but there is still enough 2-row to covert the unmalted cereals.