Add ports to existing Simulink S-Function

Is it possible to add a port to an already existing Simulink function? It has already been created with no 开发者_如何学Cin/out ports, and I can't find anything in the GUI or documentation about adding ports except when you run the add_block function.

Do you have the source code? Is it a mex file or an M file? If you do have the source code, see the S-function documentation for information on setting the number of ports. If you don't have the source code, you can't add any ports.