You can do this with the convert URL to attachments automation. It will convert file URLs to attachments in Airtable.
It can also be used in bulk for all records in a view, or to process individual records as explained here. This would give you a button you can click in Airtable to convert the URL to an attachment.
💡 Tip: This automation can also convert multiple URLs to attachments at once. Make sure to separate the URLs with commas, semicolons, spaces, or linebreaks, and include whitespace between the URLs and the separator. For example https://yourfirst.url ; https://yoursecond.url