The ISS is already basically self-sufficient when it comes to air and water. And we've grown food in space several times, so that part is certainly technologically possible.

Such a structure will still rely on parts shipped in from Earth to maintain many of their systems, though. There's just no way to produce absolutely everything you'd need without basically replicating Earth's entire economy from the ground up.

So its unlikely that such a structure will ever be a completely self-sufficient ecosystem. At least, not until we invent a Star Trek-style replicator!