The documentation states:
A notification’s big view appears only when the notification is
expanded, which happens when the notification is at the top of the
notification drawer, or when the user expands the notification with a
gesture.
So my answer is no, you can’t expand it by default.
There is however a trick to push the notification to the top of the list where it would be expanded. Simply set the Priority to Notification.PRIORITY_MAX and chances are that your app’s notification will make it to the top.