A computer-implemented technique that allows a per element mixture of “concrete” XML elements and “virtual” XML elements that are generated dynamically from external data sources. The technique extends the XML Schema language with declarations of how additional substructure is injected into existing instances. The instances created according to an XML schema with such extra declarations—called pseudo-elements and pseudo-attributes—thus mix original XML structure with the injected structure, but without creating a complete XML instance. The consumer of the structure cannot distinguish between the original and injected parts except by reading the XML Schema containing the declarations.