I would think you could put in the variable spring on the piping system segment, say node 20 on element 10-20, with a CNODE to another element, say node 1000 on element1000-1010.

That element would be a rigid of minimal length not connected to the model. You would model your constant spring on node 1000, and if you really wanted, CNODE it to a node on a structural model.

That's how I would attempt it. Whether Caesar will actually allow that to work, I don't know.