In this paper, we develop a modular design method of decentralized controllers for linear dynamical network systems, where multiple subcontroller designers aim at individually regulating their own control performance with accessibility only to respective subsystem models. First, from the standpoint of a single subcontroller designer who manages his own subsystem, we derive a constrained version of the Youla parameterization that characterizes all retrofit controllers, defined as an add-on type subcontroller such that the resultant feedback system is kept robustly stable for any variation of neighboring subsystems, other than the subsystem of interest, as long as the original system before implementing retrofit control is stable. Then, we find out a special internal structure of the retrofit controllers under the supposition that the interaction input signal coming from neighboring subsystems is measurable. We further show that the simultaneous implementation of multiple retrofit controllers, designed by individual subcontroller designers, can contribute to improving entire control performance in the sense of an upper bound. Finally, its practical significance is demonstrated by an illustrative example of frequency regulation with the IEEE 68-bus power system model.