Based on the append to the main post - you seem to be looking for a formatted file that can be opened int Excel.
It is more advisable then to convert the entire file to an HTML table, with your particular lines been converted to bold.
If it is a simple CSV ( with no commas used purely as separators and not occuring within the columns itself) then you can do the following to convert it to an html.
sed 's/^/<tr><td>/g; s/$/<\/td><\/tr>/g ; s/,/<\/td><td>/g ; $s/$/<\/table>/ ;1s/^/<table>/' yourfile.csv > yourfile.html
Remember this will convert it only to an HTML file that can be opened in Excel.
To bold a specific line you can add the following command to the set of sed commands (lets say you want to bold the 5th line)