In the toolset, choose "File", "Export", then click the "Add Resources" button. This will open up an export window. Under the "Resources of Type" you'll find a pull down window. Set it to the type of resource to export. This'll display all the resources of that type by resref in the main window. Just highlight them all, then select export. If you wish to lump creatures, items, placeables, etc... into a single erf, you'll either need to navigate back and select each grouping or export one group at a time and append the export file with new additions.