Is it possible to add a virus scanner for emails.
I think, then a email proxy is needed.
A virus scanner for emails is not that great if it cannot block encrypted attachments, or unzip archives etc, and give admins and users a portal in which they can view potentially blocked mails safely, and potentially restore them.
I would rather use a mail transfer agent (MTA) service that offers these features instead if relying on clamav and postfix on opnsense.
You need at least a well configured amavis/spam-assassin/clamav environment to get some benefit, moreso with a professional paid solution.
If you just have a few clients, the virus scanning software on the PCs will already do the heavy lifting where it counts.
To add to this, all that can be done with opnsense as it is, there is postfix, rspamd and clamav.