The CELAS2 spring element do not support preload with NX NASTRAN BASIC Nonlinear Analysis (SOL106).
But you always can preload a CROD element using an equivalent thermal load approach, or with NX NASTRAN ADVANCED NONLINEAR solver (SOL601) you can use a CBAR/CBEAM element that support directly BOLT PRELOAD.
I tried it out in NX12, but it didn't work. And I have the same Problem to define a preloaded spring. Is there any other possibility to create it?
It is not clear if you want a preloading force acting on the spring, or if you want to compress the spring with an enforced displacement elongation.
Maybe you can use three different strategy:
In general I suggest to use a low-stiffness spring in order to get a small variation of load if the structure moves a little bit.