What's the adjective for addend? Here's the word you're looking for.
Included below are past participle and present participle forms for the
verbs add, addend and adde which may be used as adjectives within certain contexts.
additive
(mathematics) pertaining to addition; that can be, or has been, added
“Elements of these programs have generally been described individually, but the additive effects of various types of intervention have rarely been studied.”
“His signature is a plastic syringe filled with addable jam stabbed into the heart of each dough ball.”
“But the Eskimoan language group uses an extraordinary system of multiple, recursively addable derivational suffixes for word formation called postbases.”