In Configuration, one can create a maniphest.priorities object with a negative key (i.e., a priority lower than Wishlist). However, when trying to actually create a task with that priority, the following AphrontQueryException is shown:
#1264: Out of range value for column 'priority' at row 1
Any particular reason they should be nonnegative? This is annoying because there is thus no way to create a priority lower than "wishlist". If I bump up wishlist's priority to (say) 10 and put my new priority ("dreaming") as 0, then all the things that were already "wishlist" become "dreaming", which I don't want.
You can safely:
SELECT * FROM phabricator_maniphest.maniphest_task WHERE priority = 10;
Make sure that's empty (i.e., your target priority is unused), then:
UPDATE phabricator_maniphest.maniphest_task SET priority = 10 WHERE priority = 0;
If you have ~hundreds of tasks, you can also batch-move priorities with the batch editor. After T5166, this should handle arbitrarily large numbers of tasks.