I thought that I need to use some sort of augmented designs.
Since I'm a beginner at this field, I wonder, wouldn't that (possibly?) change the optimality of generated design?
You can certainly use the augment design capability but it would not be "user defined" as you indicated. The optimality would be impacted slightly however all verification runs for that matter would have a minor impact the optimality to a certain degree. If you prefer you could add your "user defined" run manually and then run the augment design to add any additional runs to preserve your optimality but in my opinion and experience I prefer to validate the original design with runs that add value rather than preserve optimality.