A couple of weeks ago I made it possible to upload your own themes to the editor, so it makes sense to also allow you to upload your own extensions.
Extensions in Numbas can affect any part of the Numbas runtime. Typically, that means adding new functions in JME or javascript (or even a new JME data type) to avoid repeating the same code across several questions.
I’ve written some instructions on how to write an extension, along with a couple of examples, in the Numbas documentation.