To use transclusion, write like this example, in original wikitext :
<section begin=sectionname /> texte to transclude <section end=sectionname />
In destination wikitexte write like this example :
{{#lst:Template or page name| sectionname}}
see also Parser functions.
In original wikitext, we can use several sections.
Some years ago, I also ask to HTML the *include* way which exists in many languages. They
answer "not exists".
This question continue in the web, and all these workarounds show a miss in HTML5.
This way could be usefull :
<include src="url" from="tag.class #id" />
with *src* to any page, internal or external of the site,
and *from* to define which part to include, with the same syntax as jQuery.
Rical