Ah, I misunderstood “import”. I guess you just want to model a flag in Rhino and let it wave by means of Bongo, right?
Probably a Loft with History recording of Morphed curves will do the trick. Take a look at the Flying sample model (http://files.na.mcneel.com/bongo/2.0/samples/flying.3dm) for inspiration.
When you can give me an idea of the complexity of the waving (a video?) then I can give it a try.
Luc