It's possible to write your own macros for use in your stories. Doing so will require knowledge of both JavaScript and the Twee API, but fortunately, Twee macros are written in almost the same format as TiddlyWiki ones. The Tiddlywiki.org documentation can be very useful.

TiddlyWiki macros extend the config.macros object; Twee macros extend an object named macros directly. This is the only difference in how macros are created.

Here's an example passage that creates a macro named <<hello>> that, when invoked in another passage, displays an alert that greets the reader.

Hello macro
macros['hello'] =
{
	handler: function()
	{
		alert('Hi, reader!');	
	}
}

Start
<<hello>>

If you're curious how a built-in macro works, browsing the source code repository may be helpful, too.