FTDI has 5V from the USB, the 12V would be an external source used on the target MCU.
Essentially... RS232 is +12 and -12 level shifts. TTL serial is +5V and 0V. RS232 additionally is likely inverted which is why one can reprogram "inversion" on the popular FT232R chip used on many TTL-USB devices.
If the signal is +12 and 0V, then a resistor bridge could be utilized.
There are lots of circuits available to suit you needs. Breakout boards from Adafruit and Sparkfun, too.
Check out: RS-232 vs. TTL Serial Communication - SparkFun Electronics