Can different sections of a model have different ambient temperatures?

The scenario:
An existing 2” 150 psi steam line with a valve (closed) at the end of the line. The line is to be extended by adding a couple of hundred feet of new pipe downstream of the valve. The fabrication will be done with the existing line still in the hot position. Design pressure = 160#
Design Temp = 500º F (superheated)

The problem:
Trying to figure out how to model the new line for the case when the whole system is cooled to ambient. For Operating case 1 I will have T1=70º for the existing piping and 500º for the new piping. For Operating case 2, I wanted to use T2=70º for the new, T2= -360º. This, I figured, should give me the stresses when the existing line contracts back to it's original installed position and pulls the new piping with it. The problem is that –360º F is outside the allowed range for the material. (A53 Gr.B)
One way that could work is to have the ambient temperature set at 500º for the existing piping and 70º for the new piping. If T2=70º it should model the contraction on the existing piping.

Can anyone offer suggestions?