godan verb (-su), transitive verb
add to a written text; write in more; tack on
Used when supplementing a document, letter, or note with additional content, such as extra lines, paragraphs, or details.
レポートに結論を書き足した。
I added a conclusion to the report.
手紙の最後に一言書き足してください。
Please add a short note at the end of the letter.