As far as I know, there’s no web API to do that. I recommend you look at the code for the upload plugin, and create your own plugin that can use the Strapi Entity Service (an internal API, only for plugin code) to create a ‘compatible’ uploaded media entry for each of your existing files. Look at where the upload plugin calls its provider to ‘store’ the uploaded file and get back a completed ‘file’ object, which it then uses to create the media entry.