Possible to use USBTinyISP as an FTDI?
If all you need is serial communication with the computer then yes this can be done. However, if you need true FTDI functionality (i.e. using an FTDI driver on the computer etc.) then you need an FTDI chip.

For an example of serial communication to the computer using the SPI lines from the USBTinyISP see here. That should do for a basic FTDI serial comm port type interface, but like I said it is not actually an FTDI comm port emulator IC.