The easiest way is to use Application.Run(...). The first argument should be the function name (unadorned by any library names), followed by the arguments to the add-in function. The Application.Run() method will convert Excel COM data types (such as Ranges, variants and arrays) to the appropriate data types before calling the add-in function. It will also convert the result into a COM Variant type.
Public Function UnitConverterVBA(A As Double, B As String, C As String)
UnitConverterVBA = Application.Run("UnitConverter", A, B, C)
Here I wrapped the addin function UnitConverter() with VBA version UnitConverterVBA(), and it works.