Add all your questions as standard Moodle quiz questions and they will be exported to run as quizzes on the mobile device. Note that only questions which can be marked automatically can be exported, so any essay questions will not be exported as these require manual marking.
Essentially the question types supported are:
- any question supported by the Moodle GIFT format (with the exception of essay) can be exported to OppiaMobile
- Drag and drop questions (new to core Moodle 3.1) are also supported