You should be able to get a USB-to-Serial adaptor to show up in MATLAB. Remember to restart MATLAB to see the new port show up.
The following command can list details on the serial ports found and ones that are available:
serialInfo = instrhwinfo('serial');
You might want to make sure that your serial port shows up in the "AvailableSerialPorts" field of the "serialInfo" structure obtained above.
A word of caution though that not all USB-to-Serial adaptors emulate an on-board serial port exactly. The better the adaptor emulate the on-board serial port, the better performance (reliable and robust) you will get.
Hope this helps,