A seismic calculation is always a linear calculation, so you cannot simply calculate nonlinearly with seismicity!
However, you can execute a seismic (modal) analysis with the initial stress coming from a load case. This load case should represent the total load that you want to consider (or you can focus on the dominant load case as simplification).
In the advanced solver settings you can activate the initial stress and link it to the desired load case.
In the project settings you can activate the property modifiers.
Now you can add via the Input panel (Structure > Boundary conditions) or SCIA Spotlight 1D stiffness modifiers with e.g. Ax=0.5 on the diagonals.
Note that this will have an effect on the entire model. So if you want to calculate as well without stiffness modification for combinations without seismicity, you need to work with 2 SCIA Engineer models: one model without modifiers and one model with modifiers for the seismic analysis.