If you want to include another XML-based item in your doc, the best way is to reference it with an <xref> (inline) or <reference> (block) element. In fact, if the object is not written in MicroXML, as opposed to full XML, that is the only way to include it. So SVG and MathML, for example, must always be included by reference.
As Framemaker users well know, inclusion by reference has many advantages over embedding. When you add indirect addressing capability, the choice is really a no-brainer. So uDoc has no equivalent to the DITA <foreign> element; we consider it a Bad Idea.