When you want to add a starter app, it must be a exported project file.
This can be done through File > export app package > export select location > OK
By exporting it will be a .mpk file.
Answering your question; yes it should always be a .mpk file
What is the problem you are encountering? Is it adding the content to the private marketplace or using this content when creating a new app and using it as a template?
A little more information would help us better understand how to help you out :)