Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means append, so the verb vocab version is to append.

Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you: To append something to something else, you can just sew (そ) it onto something else.