I'm not exactly sure what you're looking for, so I'm going to answer your question in a couple of ways. First, you can access C Code and .dlls by wrapping then into an ActiveX control. Check out the KB located here for more info: http://digital.ni.com/public.nsf/websearch/0D1B7F2BCF0608B886256C67004A4B1E?OpenDocument
The second thing that comes to mind would be creating a dataplugin. More info can be found at http://www.ni.com/diadem/dataplugins.htm including a programmers guide for creating your own.
I hope these links get you going in the right direction, but if you continue to have questions, feel free to repost. Thanks and have a good one.
Adam B.
Applications Engineer
National Instruments