var dtbl = new DataTable();
for (int i = 0; i < 4; i++)
{
    dtbl.Columns.Add(i.ToString());
}
dtbl.Rows.Add("11", "12", "13", "01");
dtbl.Rows.Add("21", "22", "23", "01");
dtbl.Rows.Add("31", "32", "33", "01");
dtbl.Rows.Add("41", "42", "43", "02");
dtbl.Rows.Add("51", "52", "53", "02");

var row = 3;
var column = 3;
var lines = dtbl.Select();
var rarr = new string[lines.Length * column];

int rem = lines.Length % row;
int mr = lines.Length - rem;
for (int i = 0; i < lines.Length; i++)
{
    var arr = lines[i].ItemArray;
    var r = i < mr ? row : rem;
    for (int j = 0; j < column; j++)
    {
        rarr[i / row * row * column + j * r + i % row] = arr[j].ToString();
    }
}

var result = string.Join(",", rarr);
// 11,21,31,12,22,32,13,23,33,41,51,42,52,43,53