The easiest one is creating a non persistent object with a string inside. Pass the object containing the SSN to a datasource microflow. In the microflow create the non persistant object. Now use substring to cut the SSN value in two and use the replaceAll command (https://docs.mendix.com/refguide/string-function-calls#replaceall) to change all the number values for asteriks. Then combine the two again and place them inside the string of the non persistent object and let the microflow return that object.