In integrated process networks, the presence of large recycle flow rates induces a time scale separation where the individual units evolve in a fast time scale while the overall network evolves in a slow time scale. The slow dynamics of such networks is modeled by a high-index differential-algebraic equation (DAE) system which, in the case of cascaded control configurations, has a control-dependent state space. In this paper, a minimal-order dynamic extension is proposed to obtain a modified DAE system of index 2 with a control-independent state space that can be subsequently used as the basis for output feedback controller design. The application of this method is illustrated for a reactor-condenser network and for a two-point control problem in a high-purity distillation column.