As of Jan 2017, Carnival does not have an out-of-the-box solution for full screen pop ups; our in-app notifications are just default roll downs today. If you wanted to have some in-app messages as a standard roll down, and others as a full screen pop up, you could achieve this with custom dev work to build the custom in-app build as a pop up.
To achieve custom pop ups for some users, but not others, you could then use Message Attributes to allow the marketer creating messages to specify how it appears.
For example, you could set a message attribute for "type of in-app notification" and have 1=roll down, 2=pop up.
If this is something you're interested in doing, reach out to your CSM or Support to help schedule a call with Implementation to discuss the technical details for how this could be achieved.
For more on customizing in-app notifications: https://docs.carnival.io/docs/setting-up-in-app-messaging#section-in-app-notifications
For more on Message Attributes if you wanted the option of using two in-app notification types: https://marketing.carnival.io/docs/message-attributes