Testing in different configurations or with masses or springs added to the structure is, in the case of complex industrial structures, a time consuming and often a difficult task. The introduction of fictitious modifications by manipulation of measured receptances from an initial test represents a method with clear advantages over physical modifications on the actual test-structure. In this paper the results of different fictitious modifications applied on a combustion chamber outer casing (CCOC) are presented. A better separation of the close modes is obtained by introducing fictitious radial mass modifications on the measured receptances and the finite element model. The position and mass value are important for a better correlation of different families of mode shapes and two different modifications are presented in detail.