Working on 2 assumptions here:
1 - The project isn't VITAL that you use the usb-rs232 converter
2 - The device you're communicating with doesn't require the extra RS232 signals above TX-RX (i.e. RTS etc for control).
With both of these assumptions true, you can easily use something like this Serial-RS232 Converter which even comes with a nice DB9 connector.
This would really plug and play. Connect Serial 1, 2, or 3 to the RX-TX on the converter, as well as power, and you'll be flying.
However, if your application requires the control signals as part of the communication, then you're going to need to look at a more advanced converter. By the by, I think this is reasonably unlikely and that a cheap Serial - RS232 converter should work.
I haven't used the above linked converter before but I have used this one many many times and it's been great (once you get the TX-RX connected correctly).
Good Luck,
JW