In noise and vibration engineering, a structure's passive dynamic properties are often evaluated in terms of its frequency response functions (FRFs). The typical FRF measurement campaign consists of controlled structure excitation and the capturing of its response. However, exploiting operational excitation for FRF acquisition is not feasible, with many sources simply too complex to model or measure directly. In the present paper, an alternative approach is proposed for an indirect FRF estimation of a system in operation, in which the source is characterized independently of the final assembly. To overcome the issue of the unmeasurable excitation force, transfer path analysis (TPA) methods are proposed. TPA replicates the source excitations using the set of equivalent or pseudo forces that are an inherent property of the source. The assembly's FRFs are then evaluated on the basis of receiver responses and pre-determined pseudo forces for independent operational load cases at the source. Thus, a single source description can be applied to estimate the FRFs of any assembly with an identical source and arbitrary passive side.