The early versions of Fireworks saved symbols only to PNG images. This was achieved by saving any vector paths to a PNG image and then importing them via the document library. With more recent versions of Fireworks such as CS6, you can save the symbol or path to a common library and a different format. The Fireworks symbols common library format is .graphic file and the Fireworks symbols common library files are stored in the application support (mac) common library and any custom ones are stored in the custom symbols folder.
If it is a normal path and it is not already a symbol then you can just go to the modify menu and convert to symbol and set the enable ON and save to common library to ON. You will have to give it a name and it will be stored as a .graphic common library symbol for future use. If it is already a symbol then you can skip the convert and just select the Firework symbol in the document library and right menu and save to common library command.
Go to the custom symbols section of the common library. The one good thing about the common library is that the vectors are now available at all times and no longer need to be re-loaded via the import. Also, you can use the common library resource as a starting point for rich symbols in FW.