This is not currently possible.
Obviously, it would be sensible if it was possible. (I'm not sure if there is already an open feature request for this in the Moodle Tracker. There probably is, but if you search and really cannot find it, please feel feel free to create a feature request.)
There is reason why this is not possible: In the old days (before Moodle 2.1) it would have been completely impossible to implement because of several parts of how Moodle stored the data about which questions were in the quiz, and each student's quiz attempts. Gradually, over the years, I have been changing things to remove those problems with the database schema. (Significant changes in 2.1, 2.8, 2.9 and possibly other releases.) We may now finally be at the point where all the impediments have been removed, and it would be possible to implement this feature request without a huge amount of work.
I still have a user-interface / usability worry. Obviously it is sensible to allow the same calculated (or STACK, formulas, variablenumeric) question to be added multiple times, but not so sensible to allow the same non-calculated question to be added (though there still cases where you would want to, e.g. you might want many copies of an essay "Why did you give that answer to the previous question?"). So, how to make it possible for calculated questions, without making it too easy for teachers to accidentally make the mistake of adding the same non-randomised question when that is not what they wanted?
