1660s, "added as something extra," from supplement (n.) + -ary. Suppletory in the same sense is from 1620s.
Entries linking to supplementary
supplement n.
late 14c., from Latin supplementum "that which fills up, that with which anything is made full or whole, something added to supply a deficiency," from supplere "to fill up" (see supply (v.)).
-ary
adjective and noun word-forming element, in most cases from Latin -arius, -aria, -arium "connected with, pertaining to; the man engaged in," from PIE relational adjective suffix *-yo- "of or belonging to." The neuter of the adjectives in Latin also were often used as nouns (solarium "sundial," vivarium, honorarium, etc.). It appears in words borrowed from Latin in Middle English. In later borrowings from Latin to French, it became -aire and passed into Middle English as -arie, subsequently -ary.
supplemental adj.
c. 1600, from supplement (n.) + -al (1). Related: Supplementally. Generally "added to supply what is wanted," whereas supplementary historically tends toward "added as something secondary or supernumerary."