You can create almost anything you like. See the "user defined functions" tab in the library browser.
You can use a "MATLAB function" or "Matlab subsystem" block to embed Matlab code in a Simulink block.
You can use an "S-function" block to embed C/C++ code in a simulink block.
And a "Subsystem block" can contain any number of other Simulink blocks, including Matlab function blocks and S-functions.