As Hans points out, simple conversion is necessary. It would look similar to the following:
System::String ^str = L"Blah Blah";
pin_ptr<const wchar_t> convertedValue = PtrToStringChars(str); // <-- #include <vcclr.h>
const wchar_t *constValue = convertedValue; // <-- Unnecessary, but to be completely verbose
void std::basic_ifstream<_Elem, _Traits>::open(constValue, mode, i);