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.
Thanks for your answer.
Adding a bolt pre-load seems not to be what I'am looking for.
I want to join these two surfaces together and calculate the deformation.
I thought to add spings with 0 mm free length on the edges