Yes, of course you can use any supported storage with your free plan. Please log in to your account, select My Storage and click 'Create New Storage' button to configure S3 storage with Saaspose; see http://saaspose.com/docs/display/rest/How+to+Configure+3rd+Party+Cloud+Storage for more details.
Once S3 storage is configured following above mentioned steps, you can use 'Storage' and 'Folder' parameters with any API you use. Storage name will be the one you just configured/created by clicking 'Create New Storage' button and folder path will start with S3 bucket name. Please check RUBY examples in the documentation and let us know if you need some specific examples if those are missing.
Support Developer, Saaspose