Hi Niels,

I've actually recently had to do this myself - my code is probably not the "best" way of doing it, but here's a sample (this was in Umbraco 7.14.0).

First, the classes I serialized the JSON for the grid to:

    //we need to return something like this:
        //the hierarchy is:
        //grid
        //  sections
        //      rows
        //          areas
        /*
    "{
      ""name"": ""1 column layout"",
      ""sections"": [
        {
          ""grid"": 12,
          ""rows"": [
            {
              ""name"": ""Headline"",
              ""areas"": [
                {
                  ""grid"": 12,
                  ""controls"": [
                    {
                      ""value"": """ + escapedQuotes + @""",
                      ""editor"": { ""alias"": ""rte"" }
                    }
                  ]
                }
              ],
              ""id"": ""9feb0fd9-513d-9a92-6d15-c78c242a3cf2""
            }
          ]
        }
      ]
    }";*/

        public class GridEditor
        {
            public string alias { get; set; }
        }
        public class GridControl
        {
            public GridEditor editor { get; set; }
            public string value { get; set; }
        }

        public class GridConfig
        {
            public string Class { get; set; }
        }

        public class GridArea
        {
            public int grid { get; set; }
            public GridConfig config { get; set; }
            public List<GridControl> controls { get; set; }
        }

        public class GridRow
        {
            public string name { get; set; }
            public List<GridArea> areas { get; set; }
            public string id { get; set; }
        }

        public class GridSection
        {
            public int grid { get; set; }
            public List<GridRow> rows { get; set; }
        }

        public class GridContent
        {
            public string name { get; set; }
            public List<GridSection> sections { get; set; }

        }

//examples of how I used them to populate the grid - writer is a StreamWriter but you could just use Umbraco's own logger.

var gridContent = new GridContent
            {
                name = "1 Column layout",
                sections = new List<GridSection>()
            };

gridContent.sections.Add(
                    new GridSection()
                    {
                        grid = 12,
                        rows = new List<GridRow>()
                    }
                );


var newRow = GetGridRowFromContent(div, writer);

                        if (newRow != null)
                        {
                            gridContent.sections[0].rows.Add(newRow);
                        }

//GetGridRowFromContent is a separate method, but you will need to implement it based on your content - here's my implementation:
private static GridRow GetGridRowFromContent(HtmlNode topNode, StreamWriter writer)
        {
            try
            {
                var colSize = MatrixConversionHelper.GetColumnName("100");
                var newGridRow = new GridRow()
                {
                    name = colSize,
                    areas = new List<GridArea>(),
                    id = Guid.NewGuid().ToString()
                };

                var newArea = new GridArea()
                {
                    grid = MatrixConversionHelper.GetColumns(100),
                    config = new GridConfig { Class = "h-100" },
                    controls = new List<GridControl>()
                };


                newArea.controls.Add(new GridControl
                {
                    editor = new GridEditor { alias = "rte" },
                    value = topNode.OuterHtml.Trim()
                });
                newGridRow.areas.Add(newArea);

                return newGridRow;
            }
            catch (Exception ex)
            {
                writer.Write("Could not create a row object with error: {0}", ex.Message);
            }
            return null;
        }
    //newContent is an instance of IContent retrieved or created via the ContentService.
    var jsonGridContent = JObject.FromObject(gridContent);
    newContent.SetValue("contentGrid", jsonGridContent.ToString());