optistruct - spring stiffness definition using curve data

Hi,

I would like to know if such a spring stiffness definition, as the picture below shows (with displacement-force curve), for optistruct possible is. I found something by pbush1d, is this the only possibility to model the spring with three such 1d springs along the coord sys axes: x,y,z? 

Regards,

Szilard Balazs