The Collaborative International Dictionary of English v.0.48:

Suppositional \Sup`po*si"tion*al\, a.
   Resting on supposition; hypothetical; conjectural; supposed.
   --South.
   [1913 Webster]

WordNet (r) 3.0 (2006):

suppositional
    adj 1: based primarily on surmise rather than adequate evidence;
           "theories about the extinction of dinosaurs are still
           highly conjectural"; "the supposed reason for his
           absence"; "suppositious reconstructions of dead
           languages"; "hypothetical situation" [syn: conjectural,
           divinatory, hypothetical, hypothetic, supposed,
           suppositional, suppositious, supposititious]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "suppositional":
   accounted as, alleged, arguable, assumed, assumptive, at issue,
   confutable, conjectural, conjectured, contestable, controversial,
   controvertible, debatable, deemed, deniable, disputable, doubtable,
   doubtful, dubious, dubitable, given, granted, hypothetical, iffy,
   in dispute, in doubt, in dubio, in question, inferred, mistakable,
   moot, open to doubt, open to question, postulated, postulational,
   premised, presumed, presumptive, problematic, putative,
   questionable, refutable, reputed, speculative, supposed,
   supposititious, suppositive, suppository, suspect, suspicious,
   taken for granted, understood