According to the code and standard practice, stiffness reduction for DAM applies for Required Strength analysis, but for Design Strength calculation and for Service check, nominal stiffness is to be used.
To be able to indicate Staad which combinations are Service combinations so, for those, use nominal stiffness.
Thank you for posting this idea, but I am not clear what your proposed idea is? If you are suggesting that it should be possible to perform DA on some load cases and regular analysis on others, then that is already possible with STAAD.Pro.