Firefox has a similar feature. You can define your own keyword rule, for example, to macro-expand
hn duckduckgo
into
https://hn.algolia.com/?q=duckduckgo
and thus get a shortcut to HN's own site search (Algolia), without involving Google/DDG/etc. To do this, you need simply to bookmark the URL
https://hn.algolia.com/?q=%s
and set the bookmark's "keyword" property to "hn". Then "hn" becomes a special operator in the omnibar, which eats the rest of the search string and substitutes it into its bookmark's "%s" parameter.
(This isn't limited to search engine query strings: it's general-purpose template expansion on the URL string).