Never mind, I found the solution myself:
Legend legend = (Xceed.Chart.Core.Legend)(cscResult.Legends[0]);
legend.Mode = LegendMode.Manual;
LegendDataItem item = new LegendDataItem();
item.Text = "Some text";
item.MarkFillEffect.Color = Color.White;
item.MarkShape = LegendMarkShape.Rectangle;
legend.Data.Items.Add(item);