Hook into import in backend

I have an October CMS project (version 3.6.30). Several Tailor blueprints are used to define custom content types. Among other fields, there are some “fileupload” type fields, to store images and documents. For instance, all content types have a “featured_image” field. I’m using S3 compatible object storage (DigitalOcean spaces), which is configured as Laravel storage driver and works fine. Data can be entered through the default backend forms (automatically generated by Tailor). Now, I need to import external data, by using a CSV o JSON file. The problem is that the file contains only the external URL for every image or document. For example, for the “featured_image” field, the import file will contain something like “https://somesite.com/path/to/file/image.jpg”.
Considering the previous explanation, I’m looking for some guidance to code a custom plugin, to hook into the import process and automatically upload the images and documents to the object storage, and update the corresponding “fileupload” type field in October CMS, during the import operation.

Thanks in advance.