j_h_nimrod
It has been done... I think somebody in Oregon has modified one. It is not a cheap process. TC does not offer a BP Nipple combination for one. If/when you were to build one - then the measurement to get the nipple the correct distance from the face the firing pin block would need to be very precise. The strength of the hammer spring would need to be increased to provide the firing pin enough force to set off a cap.
One of the reason that Traditions NW break opens have problems with consistent ignition from rifle to rifle is again because of the thought of converting a 209 rifle to a cap rifle. In some it works very well in others it is highly suspect.