Do you have the actual data you need already in your Byte[] array, or the whole workbook as binairy data?
If it is just the data you need, you can maybe try something like:
My_data_string = System.Text.Encoding.ASCII.GetString(My_byte_array)
I've never used this myself, so chances are that I misunderstood your question and/or point you in the wrong direction, but it is worty trying. ☺