Ok @nyalldawson, could I add two similar methods ?

    static void wellKnownMarkerToSld( QDomDocument &doc, QDomElement &element,
                                      QString name, QColor color, QColor borderColor = QColor(),
                                      double borderWidth = -1, double size = -1 );
    static void wellKnownMarkerToSld( QDomDocument &doc, QDomElement &element,
                                      QString name, QColor color, QColor borderColor, Qt::PenStyle borderStyle,
                                      double borderWidth, double size );

    static bool wellKnownMarkerFromSld( QDomElement &element,
                                        QString &name, QColor &color, QColor &borderColor,
                                        double &borderWidth, double &size );
    static bool wellKnownMarkerFromSld( QDomElement &element,
                                        QString &name, QColor &color, QColor &borderColor, Qt::PenStyle &borderStyle,
                                        double &borderWidth, double &size );