Is there a way to include xml in xml in Joomla?

[Dan YJ]

This has been bothering me for some time. 

Is there a way that I could extend default template xml file with another xml ?

Reason I am asking is, I do not want the default xml file changed but would like to give an option 

to a user to extend it. 

I tried 2 approaches 

<xi:include

and 

http://stackoverflow.com/questions/5121052/can-we-import-xml-file-into-another-xml-file

but no luck so far. 

Doing this 

<!DOCTYPE install PUBLIC "-//Joomla! 2.5//DTD template 1.0//EN" "http://www.joomla.org/xml/dtd/1.6/template-install.dtd" [ <!ENTITY extend SYSTEM "extend.xml"> ]>

is not giving the xml error so it seems that inclusion works but the output is not there 

<fieldset name="basic"> &extend; </fieldset>

and in extend.xml file i have 1 field 

Is this approach  maybe "disabled" from system side ?