Ah, ok, I think I misunderstood you. When you said later go back and add them to the Gallery, I didn't realize you meant THEN go delete them from the topic and link to the copy in the Gallery.
Ideally, yes, at some point in the future the upload form should be made modular. If you have Gallery and you're uploading an image, have it go to the Gallery and instead of the [ attach:: ] bbcode being inserted, insert the [ img= ] bbcode pointing to the Gallery image. If you have Download manager, do the same thing with a link to the file in the download manager instead of attaching directly to the post.
Unfortunately right now, IPB is not flexible enough to allow something like that without modifications to the files, which none of our components can do. Something for IPB 3.0