I don't think that's possible at the moment. It would be a useful feature, though, so it might be worth raising in the Moodle Tracker.
On the face of it, it looks as if it would be a simple change to the queue_adhoc_task() method, to avoid overwriting a "next run time" value if one is supplied. It's possible it was implemented the way it is deliberately to prevent some other problem, though, so it's maybe not that simple.
