If UTF-8 is unicode, and you want just the bytes of the ASCII characters, you can do this (built in conversion in VBA):
Code:
Dim bString() As Byte
Dim strSource As String
Dim i As Integer
strSource = "ABC" ' strSource contains 65 0 66 0 67 0
bString() = strSource ' bString contains 65 66 67
For i = LBound(bString) To UBound(bString)
bString(i) = bString(i) + 1
Next i
strSource = bString() ' strSource contains 66 0 67 0 68 0
Debug.Print strSource ' Prints "BCD"