Suppose you assign additional reviewers, change the due dates, and your reviewers write they are receiving the error message  "Peer review is not active at this point for this article"

What has happened is that your editor submitted a decision/recommendation letter.  Once this happens, reviewing ends.

At this point, unassign the new reviewers so that their not providing a review is not counted by the system and conduct the remainder of their reviewing outside the system.  Email them the paper and ask for their suggestions via email.