The easiest way to profit from the new multi-dimension and multi-attribute site format in GRASS 5.0 is to create the file yourself. You can find details about the format in the s.in.ascii man page. Once you have created the file, you can "import" it into GRASS by copying it directly into your mapset: cp NameOfYourSitesFile $GISDBASE/$LOCATION_NAME/$MAPSET/site_lists/ (you have to be in a running GRASS session for this command to work).
For site import from other formats use the appropriate s.in.* module (replace the * with the name of the format you wish to import).