What is the context here? Are they adding the eggs to something, or aare they doing arithmetic with eggs? I answered "They are increasing the eggs," which of course makes no sense, but it accepted it. What sense of "adding" does "-ongeza" convey?
"Adding" and "increasing" both seem good translations of "-ongeza" to me, e.g. adding eggs when making a cake. (If the cake fails to rise, try increasing the eggs next time.)
The verb "replenishing" is about replacing supplies - not the same thing at all.