Can I covert a 3.3v/5v USB TTL Serial device to 1.8v using a LM1117 1.8v regulator?
Pin 4 of the PL2303 or FT232R SOIC package is VDD_325/VCCIO, the Serial level voltage input. If you inject 1.8V there, the serial UART pins will be at 1.8V TTL levels.

http://www.prolific.com.tw/UserFiles/files/ds_pl2303HXD_v1_4_4.pdf
http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT232R.pdf

The UARTSBee V4 uses the FT232R or a clone.

http://wiki.seeedstudio.com/UartSBee_V4/

By removing the 3.3V/5V switch on the lower right corner, and injecting 1.8v to the middle pin, you could make this a 1.8V serial adapter.