I agree with Burkhard Leuschner that the term "morpheme" is being
used to mean "lexeme," a unit with word-level meaning but which
may be spelled 'with spaces' (like two or more words). The term
lexeme also refers to single meanings; thus "bank" (the vault) and
"bank" (the river wall) are two lexemes. "BE supposed to" would be
one lexeme.
 
I think that the problem here is that tradiitonal grammar does not
have the sophistication to handle "BE supposed to" (as well as many
other constructions). The value of traditional grammar lies in its
simplicity, but that is its weakness, too.
 
--Bill Murdick