Pseudo-ensemble simulations and Gibbs-Duhem integrations are formulated within the framework of the expanded grand canonical ensemble. Pseudo-isobaric-isothermal simulations are proposed in which volume moves are replaced by fluctuations in the number of molecular segments. For large systems of dense athermal polymers, this pseudo-isobaric-isothermal method is shown to achieve mechanical equilibration faster than both conventional volume moves and the recently proposed slab volume moves. Pseudo-ensembles are also discussed for Gibbs ensemble simulations and canonical simulation (of the chemical potential). It is shown that coexistence curves for pure homopolymers and polymer mixtures can be traced by performing a numerical integration of the Gibbs-Duhem equation based on (expanded) grand canonical simulations. The validity of the methods is demonstrated by tracing the vapor-liquid coexistence cure of pure square-well chains and the liquid-liquid binodal curve of a blend of square-well chains.