supplement
sup-ple-ment
noun
An extra item added to improve, complete, or enhance something.
"The newspaper included a free supplement on travel."
verb
To add something extra to improve, complete, or enhance something.
"He took a second job to supplement his income."
Thesaurus
Synonyms: accessory, add-on, appurtenance, addendum, postscript, affix, add on, append, supplementation