请问怎样把字符串转换为BIG5碼

Private Sub Form_Load()
Dim filename
filename = "文件.txt"
Set Stm = CreateObject("ADODB.Stream")
Stm.Type = 2 '2-文本模式，1-二进制模式
Stm.Mode = 3 '3-读写，1-读，2-写
Stm.Charset = "gb2312" '初始文件编码，支持Unicode,utf-8,ASCII,gb2312,big5,gbk
Stm.Open
Stm.LoadFromFile filename
Readfile = Stm.ReadText
Stm.Close
Stm.Charset = "big5" '目标文件编码，支持Unicode,utf-8,ASCII,gb2312,big5,gbk
Stm.Open
Stm.WriteText Readfile
Stm.SaveToFile (filename), 2 '2可省略
Stm.Close
Set Stm = Nothing
End Sub