The other provided solutions are great when you want to add only 2 byte arrays, but if you want to keep appending several byte[] chunks to make a single:
byte[] readBytes ; // Your byte array .... //for eg. readBytes = "TestBytes".getBytes();
ByteArrayBuffer mReadBuffer = new ByteArrayBuffer(0 ) ; // Instead of 0, if you know the count of expected number of bytes, nice to input here
mReadBuffer.append(readBytes, 0, readBytes.length); // this copies all bytes from readBytes byte array into mReadBuffer
// Any new entry of readBytes, you can just append here by repeating the same call.
// Finally, if you want the result into byte[] form:
byte[] result = mReadBuffer.buffer();