Is it possible to auto-assign compensation changes, for example salary increases, to all employees that are covered by a trade union agreement?

Yes. Compensation Managers should use the section "Trade Union Agreement Rules" when designing the process. Note that after the process has been launched and the values (amount or percentages) have been automatically assigned to all employees covered by the relevant trade union agreement, Proposers, Reviewers, and Compensation Managers will be able to replace the default values if they choose.