This plugin is using WordPress standard methods so the Glossary term are classic Custom Post Type.
It is possible to use any WordPress plugin that add the CSV import support to custom post types. As example the WordPress Importer: Import any XML or CSV file to WordPress plugin add this support.
After uploading the file is important to define the post type that is
Glossary that will upload the terms inside WordPress.
Every plugin as a custom schema but usually the first column is the title and the second is the content.