title_body: How can I add irregularities to a morphological paradigm?
upvoted_answer: One possibility: perhaps your "-a- for plural" derives from an ancient word for "many" that was of a different declension, having different endings, so you could posit:
  
  S  -o / -og / -ot / -om / -ol 
  vs
  P  -a / -az / -asis / -amis / -ay
  
  Another possibility: the historical importation of a foreign declension pattern into words of certain classes, which practice later infects native words of other classes either in whole or in part.
  
  Consider the Latin words that have Greek declension and extrapolate from there.
  
  A third possibility: the later "regularisation" of ancient defective words actually introduces an irregularity. For example, your modern word for "mob" might be an ancient plural form of "person", so you could posit:
  
  S -a / -ag / -at / -am / -al
  vs 
  P -a / -az / -asis / -amis / -ay