List admins can configure and add subscribers to not-yet activated list
As subject says, list admins can add subscribers to a pending list. I vaguely remember this coming up with the previous version of sympa, and we were able to tweak the setting. This ticket made me aware of the problem: https://user.riseup.net/ticket/473182 (and, at least currently, this list's status is 'not-yet-activated', and has ~20 subscribers.) Apparently the subscribers even got a welcome message to the pending list.
(from redmine: created on 2010-02-23, closed on 2010-10-31)