In order to test some rejection emails to be sent by the sistem I need to simulate that a SC is rejected.
Is there a way to do this step manually (write a rejection reason, reject the step, etc) ? At the moment it's done by email and I can't use the email in the dev/test system (not that it doesn't work, it's that I am not allowed).
as I can see you have submitted this question in the workflow area too, so I suspect that your questing is related to some kind of document rejection and not to SOCO reject.
Actually in the workflow you can try to run the setrejected method in the SWO1 transaction. Please note that this can be used only for your own risk in a test system. The generated document can become inconsistent.
An other option with which you can check is to search for a document, where the decision was already carried out. In this case you can trigger the mail sending again. For this you can use the wiki page: