using System;

using System.Linq;

​

static class Program

{

    public static void Main()

    {

        // this is my string

        string s = "7C04048404048C04049404059C0405";

​

        // this is my byte array to which I want to append the string, keeping the same format of the array

        var bufferByte = new byte[] { 0x40, 0x03, 0xE8, 0x03, 0xE8, 0x49 };

​

        bufferByte = bufferByte.Concat(Convert.FromHexString(s)).ToArray();

​

        Console.WriteLine(Convert.ToHexString(bufferByte)); // 4003E803E8497C04048404048C04049404059C0405

    }     

}