code: private static Geometry insertVertexInMultipoint(Geometry g, Point vertexPoint) {
          ArrayList<Point> geoms = new ArrayList<Point>();
          for (int i = 0; i < g.getNumGeometries(); i++) {
              Point geom = (Point) g.getGeometryN(i);
              geoms.add(geom);
          }
          geoms.add(FACTORY.createPoint(new Coordinate(vertexPoint.getX(), vertexPoint.getY())));
          return FACTORY.createMultiPoint(GeometryFactory.toPointArray(geoms));
      }
docs: Adds a Point into a MultiPoint geometry.
  
  @param g
  @param vertexPoint
  @return