There are no registered handlers for Plays JSON implementation - you could add handlers to automatically translate plays Js Types to BSON types. However, that wont handle mongodb extended json which has a special structure dealing with non native json types eg: date and objectid translations.
An example of using this is:
import com.mongodb.util.JSON
val obj: JsValue = Json.obj("age" -> JsNumber(100))
val doc: DBObject = JSON.parse(obj.toString).asInstanceOf[DBObject]
For an example of a bson transformer see the joda time transformer.