Take a look at the Enforced support option in STAAD.pro. With this you can force a specific displacement on a node. Internally this is always used on conjunction with a support that is rigid in that direction, however.
To put some springiness into the system you can model another member parallel to the direction of the spring (like a stub perpendicular to a beam) and then apply the enforced displacement at the far end of that stub. The length, area and E value of the stub determine it's springiness.
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Multi-linear springs might be another option.