I've looked the web for csv2kml and its usage guide, but I couldn't find anything valuable, even if the software supports linestring option in its interface. However, after the search query has been adjusted a little, I've found out some examples of how the linestrings are created and how they should look in your files.
Take this example:
Duration, user, lat-start, long-start, lat-end, long-end
and it should look like this:
298;Casual;LINESTRING(-77.0444 38.9101,-77.03818 38.91554)
You can adjust this format per your needs in the files that you want to convert.