Encoding 

you can encode entire objects by defining instances of EncodeBson

//a junk trait
trait Junk {
  val x: Int
  val y: String
}
//define an encoder using the apply method in the EncodeBson object.
implicit def junkEncodeBson: EncodeBson[Junk] = EncodeBson { j =>
  DBO("x" :> j.x, "y" :> j.y)
}
val junk1 = new Junk {
  val x = 133
  val y = "some stuff"
}
val dbo: DBObject = junkEncodeBson(junk1)
//alternatively
val dbo: DBObject = junkEncodeBson.encode(junk1)

alternatively, using an implicit class in Builders that has so far gone unmentioned - ValueToBson, you could replace the last line there with

val dbo: DBObject = junk1.asBson 

(asBson looks for an implicit encoder instance for the wrapped type, and uses its encode method)

if you have an EncodeBson trait for some type, it can also be used whenever an EncodeBsonField is needed.

val dbo: DBObject = DBO("isJunk" :> true, "junk1" :> junk1)

you could replace the definition of junkEncodeBson using one of the bencode methods defined via EncodeBsons.

implicit def junkEncodeBson: EncodeBson[Junk] =
  bencode2f((j: Junk) => (j.x, j.y))("x", "y")