There is some good information at
Paphiopedilum & Phragmipedium Intro Page - AnTec Laboratory go to the the paph reading room and find the article on supplementing calcicolous paphs.
That said I top dress all my pots with a mix of granular lime and crushed sea shell. Its main purpose is to buffer the media and keeping it from going too acid. Marble, limestone, seashell, egg shell and lime are all Calcium Carbonate in diffeant forms. Some just disolve easier than others,