Создайте методы в вашей DLL, которые имитируют c функции System.Math.
Public Class MyDLL
Public Shared Function Sin(AngleInRadians As Double) As Double
Return System.Math.Sin(AngleInRadians)
End Function
End Class
edit: Макс. С использованием объектов
Public Shared Function Max(val1 As Object, val2 As Object) As Object
'tested with
' Dim ii As Integer = CInt(MyDLL.Max(7I, 2I))
' Dim dd As Double = CDbl(MyDLL.Max(7.3R, 2.42R))
Select Case True
Case TypeOf val1 Is Double
Return System.Math.Max(DirectCast(val1, Double), DirectCast(val2, Double))
Case TypeOf val1 Is Integer
Return System.Math.Max(DirectCast(val1, Integer), DirectCast(val2, Integer))
'etc - add others
End Select
End Function