WordNet (r) 3.0 (2006):

nonmandatory
    adj 1: not required by rule or law [syn: nonmandatory,
           nonobligatory]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "nonmandatory":
   arbitrary, autonomous, discretional, discretionary, elective, free,
   free will, gratuitous, independent, offered, optional, proffered,
   self-acting, self-active, self-determined, self-determining,
   spontaneous, unasked, unbesought, unbidden, uncalled-for,
   uncoerced, uncompelled, unforced, uninfluenced, uninvited,
   unpressured, unprompted, unrequested, unrequired, unsolicited,
   unsought, voluntary, volunteer, willful