Astra Starter Sites plugin does not delete existing content, but it imports new settings and content.
More specifically, the plugin imports things as below:
1. Customizer Settings
3. Menus and Content (Pages, Posts, Images and Media)
4. Custom settings of some plugins (like Elementor, Beaver Builder, Astra Pro, etc.)
So you can import an Astra Starter site on existing website. But as the process imports new content and settings as well, just go to pages, posts, menu, widgets and delete the content that was added in the import process.
Just to be safe, take a backup of your existing website. It’s easy and takes only few minutes.