ASP Gridview (put more items in each row)?

You need to get the data source from your gridview and store the data source in a data table.
Add your new rows to the data table.
Set the data source of the grid view to the modified data table and bind your data.

DataTable dt = (DataTable)GridView1.DataSource;
object[] obj = new object[4];
obj[0] = "Column 1";
obj[1] = "Column 2";
obj[2] = "Column 4";
obj[3] = "Column 3";
dt.Rows.Add(obj);
GridView1.DataSource = dt;
GridView1.DataBind();