int column = 5;
string[] lines = File.ReadAllLines(@"E:\a.txt");
string[] result = new string[lines.Length * column];

for (int i = 0; i < lines.Length; i++)
{
    string[] arr = Array.ConvertAll(lines[i].ToCharArray(), v1 => v1.ToString());
    for (int j = 0; j < column; j++)
    {
        result[j * lines.Length + i] = arr[j];
    }
}