Dear Jason Teunissen,
The only way to do this is to create a NP entity with string attribute and pass it to sub microflow and do changes and pass back and you don't need to change again in main microflow. Except object we can't change primitive data types as the parameter is read only. And the created variable inside a microflow is like a private variable and can't be accessed or modified inside another microflow.
Hope my answer helps, reach me out for any quries.