Additional data in ModelData.Testsockets[0].UUT

I'd like to store additional data in the UUT. I have created the variable called Reference but It seems that I need to create a new UUT (Container) Type and replace it in the array. How can I do that? Shall I create a whole new model? 

Parameters.ModelData.TestSockets[Float64(Locals.CurrUUTData.Slot)].UUT.AdditionalData.Reference = Locals.currUUTReference. 

gives 

The post-expression for the step 'Statement' could not be evaluated.
Unknown variable or property name 'Parameters.ModelData.TestSockets[Float64(Locals.CurrUUTData.Slot)].UUT.AdditionalData.Reference'.

currUUTReference is reference to the UUTClass object.