To add something to what has already been said; to say an additional remark or supplement.
重要な点を言い添えると、締め切りは明日の午後です。
To add an important point: the deadline is tomorrow afternoon.
報告書に一言言い添えて提出したほうがいいと思う。
I think it's better to add a brief remark to the report before submitting.
彼の説明に言い添えておくと、計画にはまだ予備費が必要です。
To add to his explanation: the plan still needs contingency funds.