If you want to use Liquid within other files (such as feed.xml or pages.json), just create the file as you normally would, but append .liquid to the filename.
For example, if you wanted to create a JSON file with all of your blog posts, you would create a blog.json.liquid file (the filename can be whatever you like). Within this file you could use the json Liquid filter to properly format your posts in JSON. When you publish your Siteleaf site, a blog.json file is automatically created for you.