  In this paper we are concerned with the study of additive ergodic averages in
multiplicative systems and the investigation of the "pretentious" dynamical
behaviour of these systems. We prove a mean ergodic theorem (Theorem A) that
generalises Hal\'asz's mean value theorem for finitely generated multiplicative
functions taking values in the unit circle. In addition, we obtain two
structural results concerning the "pretentious" dynamical behaviour of finitely
generated multiplicative systems. Moreover, motivated by the independence
principle between additive and multiplicative structures of the integers, we
explore the joint ergodicity (as a natural notion of independence) of an
additive and a finitely generated multiplicative action, both acting on the
same probability space. In Theorem B, we show that such actions are jointly
ergodic whenever no "local obstructions" arise, and we give a concrete
description of these "local obstructions". As an application, we obtain some
new combinatorial results regarding arithmetic configurations in large sets of
integers including refinements of a special case of Szemer\'edi's theorem.
