I cannot see a way to do this

if the email has gone above the kill threshold then it will be gone for good. You could vote for this RFE
Bug 14167 - Improved spam filtering and reporting and ask for additional user reporting.
The alternative, which I do myself, is to put a second Postfix instance infront of Zimbra and let that perform the AS/AV function. For reference I use a combination of
::: Official Home Page for MailScanner - Anti-Virus and Anti-Spam Filter ::: and
http://mailwatch.sourceforge.net/ . Obviously this means that ZCS is not integrated with it, but it does allow for user based quarantine reports etc.
Personally I would like to see MailScanner being used instead of Amavis as IMHO it offers a greater degree of control, and would be relatively easy to integrate.
Hope that helps.