This is possible but it is not obvious. You can can set a property of the MATLAB Function Block if you happen to know just the right property name; you would need to set it to a string that contained the entire code of the function.
Alternatively you can unzip the model definition and edit the right XML file to insert the code, save, zip back up.
I do not happen to remember the correct property name at the moment. I have not done this much at all myself. I think I posted the property name about a year and a half ago, perhaps more, in response to a similar question. Unfortunately it is not as simple as just telling Simulink the name of a file to link to: Simulink embeds the code in the model.
... All of which implies that by far the easiest method is just to put in the appropriate function header and initialize the size and types of the return values, and then call out to your function.
