I would think the same method used to collect each floor's response spectrum could also be employed to develop the floor's "zero frequency" deflection. (CAESAR II uses the acceleration of the lowest spectrum frequency calculate this displacement - displacement=acceleration*omega squared, where omega is that lowest frequency in radians.)
Enveloping all the floors into a single response spectrum will eliminate these pseudo-static floor displacements but I cannot say that this is conservative or appropriate.
Also, this displacement is not applied to each node. It is applied to all boundary conditions (e.g., anchors, restraints, displacement sets, hangers, etc.) on that floor.