The only way to obtain a Munchlax in Pokémon BW is by trading a Cinccino to somebody in Undella town when its summertime, so no.
Source
>During the season of Summer, you can trade a Cincinno with a person in one of the houses in Undella Bay with a Munchlax holding Leftovers.
If you mean ingame trading, then no. However if you mean local trading the you are good to go!
Hope I helped!
