Puedes tomar el valor del separador decimal de la configuración regional y reemplazarlo en la cadena:
CadenaNueva = Replace(CadenaAnterior,GetLocaleDecimalSep,".")
Public Const LOCALE_SDECIMAL = &HE
Public Declare Function GetUserDefaultLangID Lib "kernel32" () As Integer
Public Declare Function GetUserDefaultLCID Lib "kernel32" () As Long
Public Declare Function GetLocaleInfo Lib "kernel32" Alias "GetLocaleInfoA" (ByVal Locale As Long, ByVal LCType As Long, ByVal lpLCData As String, ByVal cchData As Long) As Long
Public Function GetLocaleDecimalSep() As String
Dim strBuffer As String
strBuffer = String(255, " ")
GetLocaleInfo GetUserDefaultLCID, LOCALE_SDECIMAL, strBuffer, 255
GetLocaleDecimalSep = Trim(Replace(strBuffer, Chr(0), ""))
End Function