There is no tool provided to convert .m into Simulink .
What you can do is use a MATLAB Function Block in simulink, and have that block call your code.
You will probably have to rewrite the code a bit. Anywhere that you have a call such as
result = outerFunction(InnerFunction(inputs))
you will typically need to rewrite that like,
temporary_result = zeros(expected_size_of_inner);
result = zeros(expected_size_of_outer);
temporary_result = InnerFunction(inputs);
result = outerFunction(temporary_result);