At first thank you very much for answer!
Yes, absolutely agree with you and actually I've stopped on the stage 2.
The code fragment is following:
/* Getting point to the BarcodeValue field from the Secondary datasource
XPathNavigator myNavigator = this.Datasource["MySecondaryDatasource"].CreateNavigator().SelectSingleNode("q:_dlc_BarcodeValue");
But I've stacked on the moment how to make a query (based on , i.e. unique date for each SharePoint library row. I've made custom submitting: date from the form is publishing in appropriate field in SP lib)