You can load a CUIX file via the acedLoadPartialMenu function. In ObjectARX 2012 this function is declared in aced.h. The declaration isn't there in ObjectARX 2011 but the export is actually provided by acad.lib and acad.exe. You'll just need to declare the function yourself:
/* Loads a partial menu file. * Returns true if partial menu is successfully loaded. */ bool acedLoadPartialMenu(const TCHAR* pszMenuFile);
I haven't tested this in AutoCAD 2011 but I know it does work in 2014.
There is also an acedUnloadPartialMenu function you might find useful:
/* Unloads a partial menu file. * Returns true if partial menu is successfully unloaded. */ bool acedUnloadPartialMenu(const TCHAR* pszMenuFile);