The option “Custom incoming email address” is not available for automatic groups. That is fine for everyone and the trust_level groups, but I’m not sure it makes sense for staff/admins/moderators.
I’ve had to make a separate “support” group for ours, containing the moderators. This is fine as a workaround but has a small but annoying admin burden.
Is there a strong reason for this (or IMAP for that matter) being disabled for these groups?