Word Formation Rules are responsible for the existence of well formed words. Are simple words (single, free morphemes) the result of word formation rules?
I once proposed that English verbs do not allow two or more contiguous unstressed syllables, with the exception of denominal verbs, which have the same pronunciation as the noun they are derived from. For instance, "antagonize", ANTAGonIZE, has only unstressed syllable, but if it were a noun, two contiguous unstressed syllables would be possible: *ANtagoNIZE.
But verbs referring to use of a device of the same name must be excepted from the general rule. MImeoGRAPH as a verb means to use a MImeoGRAPH, which has two contiguous unstressed syllables. To CALendar an event is to note it on one's CALendar.
So the rule of denominal verb formation does not change the number of morphemes or the stress of a word, and if you start with a single morpheme noun, you'll wind up with a single morpheme verb.
Word-formation rules are generally assumed to combine an existing element with something. The initial element in a derivation is inserted by a lexical insertion rule. However, nothing important follows from distinguishing "word-formation" and "lexical-insertion", so they can be subsumed under one general rule concept.