I don't know anything about that particular USB->serial chipset. Further, I run a Debian release on my plug (because Ubuntu support of the plug is going away at some point in the future) so I don't have direct knowledge of that Ubuntu release. Sorry.
Personally, I would build the "Prolific PL2303 USB to serial adapter driver" module; but, I'm a software person by profession so building software is ALWAYS my first answer.
Seriously, installing a new or different release will not guarantee that module will be present so, after a bunch of work, you may end up right back in the same place.
From the documentation, the intended usage of the mini-USB port is "debugging". It is attached to the JTAG port and serial port 0 on the plug. It cannot be used to plug in random USB devices as it is a dual port USB->serial device, not a USB host port. Instead, you can plug the mini USB port into a Windows/Mac/Linux host's USB port and, after the FTDI drivers initialize, use it as a serial console or with a JTAG-based debugger. I used putty on Windows to talk through the mini-USB port (serial) to install and configure software on my plug.
In general, the USB type A port (USB host) can have any USB "thingy" plugged into it. The only conditions are:
1) The USB host port needs to provide enough power for the "thingy". (putting a powered USB hub between the plug and the "thingy" solved this problem for me)
2) The currently running Linux needs to support the "thingy" with drivers and other software? (your previous question)
I have not attached a USB->serial converter in the USB host port; but, it should work if the above are met.