For Spartan-3AN, it is possible to purchase leaded (ex. FG) packages, but in the software tools the only selectable options are the lead-free (ex. FGG) packages.
In all of the Xilinx Software Tools, you can use the lead-free package for the Spartan-3AN devices, even if you have the leaded package. The leaded (FG) package will perform the same as the lead-free (FGG) package from a software tool and timing analysis perspective.
For more information on available device/package ordering combinations, timing information, and package thermal specs, please see the Spartan-3AN FPGA Family data sheet at: