Thanks for the additional information. If you are trying to send the in app message to multiple but not all account numbers, you'd need to have some way of automatically segmenting them (ie gold status or another condition). I think you could then use something like Audiences or Customer Attributes to create a segment that could be targeted. I don't believe there is a way to simply import a list of customer account numbers.
You are correct, I checked with one of our mobile experts and in-app messages are all client side so segments and audiences wouldn't work.