It is possible. Here's how I do it.
Adding a domain or user to a black or whitelist is done on the following file:
/opt/zimbra/conf/salocal.cf
Example;
whitelist_from *@yahoo.com
whitelist_from
user@company.com
blacklist_from
sea@lab.com
blacklist_from *@mcribstatus.com
After modifying salocal.cf, you must restart Amavis to take effect.
Code:
zmamavisdctl restart