Incorporating existing m-file of RHS into simulink

I am quite new on using Simulink. My problem is quite simple but I am not sure having (and googling) the best issue for it... there is it: I want to create my own equation of motion based on the 6DOFs which is a 12 non linear-equation. In fact I've already have my RHS "dx = EOM(t,x,data)" which is a classical m-file. My question is how could I use this m-file into simulink (without recreating the model) and have simulink treats it as a non-linear. I know the best way is to use wind-axe 6DOFs built-in of the aeroblockset. But I would like to treat this like an example since if we have another non linear code (m-file, C , fortran)? Is the S-function block is the only way to do it? tks