Plus+ Members on OpenProcessing can add custom javascript libraries to their sketches. 

To add an external library hosted on a domain,  click on "+ Add custom library" below the libraries section on the code options panel, and copy/paste the URL of the javascript library you would like to add.
Eg. https://urltomylibrary.com/myLibrary.js
Make sure you use an "https" secure address, otherwise browser will not load your library.
You can also upload a javascript file on the files panel, and add its filename here to add it locally. Eg. myLibrary.js