Lightroom doesn’t support manual User Order in smart collections, so there isn’t a direct way to drag uncategorized photos into a custom sequence there.

A practical workaround is to avoid relying on User Order for anything important. Instead, once you’ve decided on a sequence, select those photos and batch rename them with a numbered prefix or sequence counter. In Lightroom, use Rename (F2 / Edit > Rename) and apply a naming pattern that encodes the order.

That makes the sort order part of the filenames, so it’s much more durable and visible both inside and outside Lightroom. It also avoids losing a custom order if something changes in Lightroom or the catalog.

So, rather than switching to folders just for ordering, the more reliable approach is to keep using collections if they suit your workflow, and use filenames to preserve any sequence you care about.