I'm not sure what you're trying to accomplish. You insert a legend onto a sheet by checking the Show Legend check-box in the Format pane. The Legend will dynamically show all markers on that sheet, i.e. if you add a new marker, it will be added to the Legend, and if you remove the last instance of a marker, it will be removed from the Legend. By double-clicking on the caption of a marker in the Legend you can type your own explanation for the marker.
While I haven't tested myself, I expect it to work also for own custom markers.