This hasn't yet been explicitly answered (as far as I know), but available evidence would point in the direction of it not being directly possible. Seeing as existing mods need an update before they're compatible with BG:EE, I would at the very least expect save files to need some tinkering before vanilla BG2 will accept them, and that is if you play a mostly unmodded game. Since BG:EE will have many more non-hardcoded features than BG/BG2, several modifications made possible in BG:EE are likely to break BG2. And if it ends up working in the end, new features like the added NPC:s obviously won't carry over.
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And if it ends up working in the end, new features like the added NPC:s obviously won't carry over.