I have a robot consisting of pneumatic muscles and valves. So far the programm runs on Arduino IDE. Is ist possible to implement it into Matlab and Simulink, to get a simulation and save the sensor values? And how?
There is no direct way to control a pneumatic value or actuator directly from matlab. However you can build your logic using Simulink and then later deploy it directly from Simulink with the help of Arduino Support package in MATLAB and Simulink.
You can also get the sensor values using the same package. This documentation will give you a good idea.
Bonus - With your logic designed in Simulink you can also use Simscape Fluid to simulate your pneumatics.