Assuming you've got +5V at a sensible current available on another USB port, you can easily knock up a little circuit using a MAX860-series chip
https://www.maximintegrated.com/en/p...ps/MAX680.html
to get your 'proper' RS232 supply-voltages.
I've done this in the past when needing to use a laptop-and-USB-to-RS232 for programming commercial two-way radios. Also had to do it back in the days when plenty of laptops used custom low-power serial-port driver chips which didn't give you a proper RS232/V.24 voltage-swing.