According to the English Wiktionary, kuongeza is a derivate of kuongea ("to speak; to chat").
i removed that derivation, because i could not find it elsewhere online, it's listed differently in etymological dictionaries, and it doesn't make sense, plus the person that added it does not speak swahili and his etymologies have been questioned before: https://en.wiktionary.org/wiki/User_talk:MDCorebear
I think not. Other bantu languages have related words for adding. E.g 'ukongeza' in Xhosa/Zulu, is unrealated to speaking which is "ukuthetha"/"ukukhuluma" respectively.