I don't fully understand what you mean, but I'm assuming that you want to access some ios platform functionality that is not provided by SDL currently. You can create your own Objective-C file and do the ios specific stuff there. Just create a normal C interface for you to call from your C/C++ code. You can also call your own C functions from Objective-C code.
You can get an ios
UIWindow* window pointer with SDL_GetWindowWMInfo if you need to reference that in you Objective-C code.