Remote Attachment for Custom post Type

I have a plugin that operates with another system I've written - the other system has a store of over 100,000 images, and each website picks from the library and displays the images on the appropriate post as per usual.

The problem is - I am copying each file to each website as an attachment to the post, making each website over 10GB in size and almost unmanageable.

How can I, through PHP, remote link to a file and have wordpress still view it as an attachment for use in a gallery? It seems trivial to just use the URL instead of having it pull from the uploads directory when adding attachments during my import routines but I can't figure it out.

So - have a remote file treated as a local file from the uploads directory in wordpress?