A mass addition technique for structural parameters updating has recently been described by the authors. It uses eigenvalues of the structure, before and after it is perturbed by adding mass, to adjust selected parameters by sensitivity analysis. The technique avoids the use of noisy mode shape data and overcomes the problem of a non-unique set of parameters when eigenvalues alone are used. However, it is not suitable for a large structure because of the difficulty of adding the necessarily large perturbing mass to the existing structure. This paper presents an alternative technique. An "imagined" stiffness is added to the structure and the FRF of this perturbed structure is obtained from the measured FRF of the original structure by a simple structural modification technique. The eigenvalues of the original and perturbed structures are obtained from the measured and constructed FRFs respectively. These eigenvalues, together with the eigenvalues predicted from an analytical model of the structure, are used to adjust the structural parameters by sensitivity analysis. The technique is demonstrated by a simulated example and by an experiment on an H-frame.