While Malte is correct that there is no general way to do this directly, with a little extra effort you can do this.
You can call Mathematica from C using WSTP. See the documentation here. Also the tutorials referenced from that page will describe how to create a C wrapper function that will send any data, run any function, and return results from MMA using C.
The C wrapper function can be called from SystemModeler using an external C block in WSM. The documentation for calling C from WSM can be found in the WSM help system and also in Modelica By Example at this link within the WSM documentation center:
wsm:///ref/ModelicaByExample/behavior/functions/controller/
The overall process would be to write a small (few lines of code) C function based on the examples in the WSTP documentation that executes whatever MMA function you want using data sent in from C. Call the function from SystemModeler using the examples above. I hope this helps.
Regards,
Neil
