You can zero pad the series number in the filenames, so instead of
  dcm2niix -f p_%s ~/tst
you would run
 dcm2niix -f p_%5s ~/tst/AxAsc36mb2a/

The github webpage 
  https://github.com/rordenlab/dcm2niix
describes tools in several languages that show how you can custom rename files for more complicated combinations.