You can create a AppendInto method that will append arrays, and use Encoding.ASCII.GetBytes to convert string to byte array.
private byte[] AppendInto(byte[] original, byte[] toInsert, int appendIn)
{
var bytes = original.ToList();
bytes.InsertRange(appendIn, toInsert);
return bytes.ToArray();
}
Then just use the function
var toInsert = Encoding.ASCII.GetBytes("151219");
var original = new byte[11] { 0x01, 0x52, 0x35, 0x02, 0x50, 0x31, 0x28, 0x3B, 0x29, 0x03, 0x06 };
AppendInto(original, toInsert, 7);
Result
byte[17] { "0x01", "0x52", "0x35", "0x02", "0x50", "0x31", "0x28", "0x31", "0x35", "0x31", "0x32", "0x31", "0x39", "0x3B", "0x29", "0x03", "0x06" }