Since this word has okurigana (hiragana attached to the kanji) you know that it's probably going to be the kun'yomi reading.
Lucky for you, the kanji readings for transitive and intransitive verbs are almost always the same. Once you learn one, you know them both! This reading is the same as 加える. To help you distinguish between the two, remember that most verbs ending in わる are intransitive.
A new character will be added soon!
A monkey joined in the volleyball match.
The cost of advertising will be added to the ten million yen for the ingredients.