Starting with Kerio Connect 8, Kerio decided to drop the development of third-party AV plugins. At the same time, Kerio offers the SDK, so companies are able to develop their own plugins.

Matthias Kerstner shows in his Blog, how you can compile your own ClamAV plugin for Kerio Connect 8. And if you are using Windows 64-bit, you can download an already built package.

Read more here:
http://www.kerstner.at/en/2013/01/clamav-plugin-for-kerio-connect-8-and-up/