to add a written note; to append or write a postscript/addendum
手紙の最後に一言書き添えて送った。
I added a brief note at the end of the letter before sending it.
契約書の余白に注意事項を小さく書き添えた。
I wrote a small note about precautions in the margin of the contract.
論文に参考文献を追記して書き添えた。
I appended additional references to the paper.