The concept of "additivity of pressure" is defined by Dalton's law of partial pressures, which states that the partial pressures of the components of a mixture sum to the total pressure of the mixture [Bejan2006, p. 192]. The components are assumed to exist at the total volume of the mixture.
In order to implement Dalton's law, this connector includes pressure as a flow variable. The effort variable is volume. This implies that the effort and flow variables are conjugates of energy (not power).
See also the Amagat connector and the documentation of the Connectors package.