The easiest way to convert a byte array to a stream is using the MemoryStream class:

Stream stream = new MemoryStream(byteArray);

You're looking for the MemoryStream.Write method.

For example, the following code will write the contents of a byte[] array into a memory stream:

byte[] myByteArray = new byte[10];
MemoryStream stream = new MemoryStream();
stream.Write(myByteArray, 0, myByteArray.Length);

Alternatively, you could create a new, non-resizable MemoryStream object based on the byte array:

byte[] myByteArray = new byte[10];
MemoryStream stream = new MemoryStream(myByteArray);

The general approach to write to any stream (not only MemoryStream) is to use BinaryWriter:

static void Write(Stream s, Byte[] bytes)
{
    using (var writer = new BinaryWriter(s))
    {
        writer.Write(bytes);
    }
}