Munchlax is only going to be available in Pearl and Diamond, the next two games that Nintendo are going to produce. No one knows if it can be traded over to RSE/FR/LG or not yet.
I would advise everyone not to get their hopes up for something like that ever happening, because there's about a 110% chance that won't happen.
I don't think it would even be possible even if they wanted to, however. Remember trading from GSC back to RBY? You couldn't bring anything into the trading room from GSC that didn't exist in RBY, because the RBY games wouldn't recognize it and if this was allowed it would most likely result in bad things happening to your game.
While perhaps it would be interesting if something like that were to happen, unfortunately, it won't.