Regarding the Case -1
Lets compare your condition with the real life situation .
Take a beam whose ends are pinned and load it will load DL . After attaining a deflected shape , lets somehow lock the pinned supports or grout the end fixity and then add more load and then analyze the structure .
As far as I know , there is no such direct way to model such condition in Staad .
However, indirectly you could simulate such situation .
First , analyze the beam model (pinned ends) with Dead Load (DL) . From the out-put files or post-processing mode, note down the deflected shape and internal forces like moment , shear etc .
Now, model another structure in seperate Staad file of the same beam . But this time , model the beam whose shape is almost same as that of the deflected shape you noted down from the first case . Assign the end supports with fixed end support condition (inclined support may be opted ) , then apply the LL load here and run the analysis .
You would get the deflected value and the end forces of the new structure .
Eventually , add the deflected values and internal forces reported from both the models to know the actual result .
If the above suggestion suffice your requirement then you can use it for the Case 2 .