I would suggest you to create a custom block and it to the Simulink Library Browser. This way, you can extend the built-in modeling functionality provided by Simulink. Since you have an existing MATLAB function that models the custom functionality, you can create a custom block using a MATLAB Function block, Fcn block, or Interpreted MATLAB Function block.
- Use the MATLAB function block if you intend to generate code from your model. This block does have restrictions, the entire gamut of built-in MATLAB functions is not available.
- Use the Interpreted MATLAB function block if you don't care about code generation, this block can make use of any functions.
- Use the "Fcn" block if your m-file is trivial and contains a simple expression operating on the inputs. In this case you can type the expression directly into the block dialog and reference the input / output signals as shown in the documentation.
In short, we can create a library having MATLAB function blocks (or one from the other two options) in which you have written the MATLAB functions.
The following link is worth looking:
Hope it helps!