Just quick browsing through the 'Nexyx 3 board tutoral', it seems that you need the Digilent Adept software to actually program the FPGA. Thus, the programming file is generated by Impact, but you need to use this separate piece of programming software to program the FPGA.
So, not directly supported through Impact.
From the tutorial (covering ISE 13.2 which is more recent than 12.1; the latest would be 13.3 at this moment):
"Loading the FPGA
The next step is to transfer the FPGA bit file through the USB cable using the Digilent Adept
software (download from the Digilent website and install).
Connect your Nexys2/3 board to a USB connector on the PC.
Click on the Adept software and the Adept software should recognize the board and
FPGA and PROM devices: ....."