You can use the standard .V file extension. Within ISE open the XST properties window. Inside these properties is a check box for Verilog 2001. Enabling this box tells the tool to use Verilog 2001 type syntax and constructs.
Verilog 2001 applies only to the XST synthesis process. After XST is done, the design has been translated into the Xilinx format.