For Windows Phone 8.0 I'm not sure, but for 8.1 there is no law that can prevent that.
For 8.1 you need to envelope it in a C++/CX class and then use that class as a normal assemby added in References Project folder.
For 8.0 perhaps a C++ library for Windows Phone that can be then used by C# code.
However there is a rule: native integrated code must use only Unicode, and only has a subset of Win32 API available. They are listed here: http://msdn.microsoft.com/en-us/library/windowsphone/develop/jj207198(v=vs.105).aspx