You can define your own macros, with or without parameters.
Create a #macros section, it can be in your cmsPrefs.opml file, at the top level of an outline, or anywhere within an outline. Under #macros put the name of the macro and under that, put the macro definition.
Here's a screen shot of the macros section of the outline for the docs website.
Because there's a hello macro defined there, I can call it this way
<%hello ()%> and it will render as the string Hello there!.
There's also a macro defined called sumArgs, which returns the sum of its arguments. I can call it this way
<%sumArgs (1, 2, 3)%> which renders as 6.