A little digging in the bullet docs revealed:
btMatrix3x3::setEulerYPR  ( const btScalar &  yaw,
  const btScalar &  pitch,
  const btScalar &  roll
)

So I'm assuming you could use that to create a btMatrix3x3, and then apply that to a btTransform's origin that can be applied to the btRigidBody.

If someone could update the two commented lines with the appropriate code, it will be exactly what I want.

No one is here to just write your code for you.  You're explicitly asking people to do that.  Try and have a little courtesy, it's a forum for information sharing, not task orders.

A little digging in the bullet docs revealed:
btMatrix3x3::setEulerYPR  ( const btScalar &  yaw,
  const btScalar &  pitch,
  const btScalar &  roll
)

So I'm assuming you could use that to create a btMatrix3x3, and then apply that to a btTransform's origin that can be applied to the btRigidBody.