You can import quiz questions in the Moodle XML format. See the specifications.
You can find the link for importing below the "Add a question" button.
The advantage of Moodle XML is that it's well documented and there are tools that can help you convert your quiz questions from other question formats into Moodle XML.
Some tools for converting questions:
Quiz questions also get exported in the Moodle XML format.
By default all questions get exported, but you can also specify to only export individual questions that you might want to reuse in another quiz.
Please note that the quiz settings don't get exported.