How can I add userfields on form in “Sales UOM and Package Types” and let the matrix controls it like it does with “Qty per pakage” ?

I’m using C# to create fields. This form has FormType 1470000010 and I aber to create event and controller classes and add fields binded with ITM4, only get the first register and bind to it (I tryed to set value in dbdatasource “ITM4”, passing offset, but seems I cannot change not user defined data source.

Here the screen below as example (forget the title form, my focus is on Sales UOM and Package Types, that is the same form). When select a Package type, the default fields works fine and saves temporary tha values after update in “Sales UOM and Package Types”

Screen image

What’s is the ideal way to add it and link to?
I tryed too to create 2 userdatasources, and listen to matrix click and edittext Lost focus to update the values using dbdatasource with “ITM4″ and get value passing offset as nextselectrow of matrix. It works, but just for the moment, I”ll need to create a datatable in this scren or Master Data Item, not sure what is best, to save the result and after update document, update the user fields (this could solution, but I’m not sure if i’m losing a more simple way).