I have a system where first anchor support in vertical line is derived from a big LNG tank. In fact from tank nozzle at tank top this line runs downward and is supported from the tank body which has negligible thermal growth. The first 3 way stop in vertical on this line is where my scope terminates. Between this 3 way stop and nozzle vendor is analysing. In my scope there are guides also on the vertical run . The three way stop , these guides all are derived from tank ( with negligible thermal growth at tank ).
Further this pipe runs down ward and after few meters runs on sleepers.
I am anaylsing as per response spectrum for seismic qualification. Vendor has provided seismic displacements at this three way stop and other guides as under :-
Seismic action in horizontal - OBE values +/- 28 mm
Seismic action in vertical - OBE values +/- 17 mm
How to input these in response spectrum at three way stop ?

Assuming you are going to use an "envelope" approach, all of the supports in the model will be loaded with your spectrum.

Already the spectrum is applied to piping model in response spectrum module. My question is further to that in this module how do I superimpose the displacement values ? Shall I use ISM option ? If yes, then please specify .

Yes, use the last column in the spectrum to explicitly define the magnitude (not direction) of the support motion associated with the direction of the defined spectrum. [Don't leave it blank unless you know what you're doing.] And throw the associated switch in Control Parameters.
Check your work by confirming the expected support displacements in the shock output.