Translation:This coffee becomes sweet with the addition of sugar.
extra is given as one of the meanings of 'tambahan' - it should be accepted as a translation
Tambahan is a noun. Extra,when used as a noun is a acceptable translation for tambahan. Eh.g This sugar is extra. Extra sugar is extra used as an adjective, hence, tambah. Kopi ini menjadi manis dengan tambah gula = This coffee becomes sweet with extra sugar.