To the best of my knowledge, this can be achieved by the LOAD ENVELOPE command in Staad using the AISC UNIFIED code.
Lets say you define :
PER ANA
DEFINE ENVELOPE
1 TO 10 ENVELOPE 1 TYPE service
50 TO 100 ENVELOPE 2 TYPE STRESS
END DEFINE ENVELOPE
LOAD LIST ENV 2
PARAMETER 1
CODE AISC UNIFIED 2010
METHOD LRFD
TRACK 2 ALL
DFF 100 ALL
CHECK CODE ALL
This means this will do only a deflection check based on load cases 1 to 10, stress check will not be done here.