Synonyms for Supplemental:

adj.

accompanying (adjective)
adjoining, additional, ornamental, companion, extra, secondary, besides, Adorning, in addition to, attached, along with, affixed, auxiliary.
additional (adjective)
auxiliary.
all (adjective)
supplementary.
relational (adjective)
ancillary, akin, relevant, coincident, correlative, connected, kindred, germane, dependent, pertinent, consanguine, concomitant, analogous, related, associated, affiliated, attendant, auxiliary, blood, adjunct.

n.

supplemental (noun)
auxiliary, added, additional, secondary, supplementary, subsidiary.

Other synonyms:

Other relevant words:
added, supplementary, additional, subsidiary.