It is possible, though not in the same way it's currently done on the web. You can certainly define and place specific ad units in your interface, and then develop creatives that work with those placements to form a unified presentation. With that done, you would just need to set each ad unit up with a single line item pointing to the correct creative for that placement. As long as you have a single line item for each Ad Unit, you would be guaranteed to achieve your desired result.
This works well if you're assigning all your impressions over a period of time to that particular set of creatives. If you'd like to market a specific number of impressions (similar to the way master/companion roadblocks work on the web), that's something for which we don't currently have a solution. We're constantly looking for ways to improve Mobile Ads, though, so it may be incorporated in the future.
-Andrew