Hey there! There's a few different ways to do this. Sometimes there are packages specifically for this, as mentioned here:
Introduction There is an RPM package named clamav-unofficial-sigs which can be found in the EPEL repository of CentOS. This package is not provided by, supported by, or managed by cPanel. Altho...
This forums post explains how to tweak existing definitions:
Hi, Can someone tell me where can I find ClamAV signature databases to exclude some rules? Kind regards.
and this explains how to perform a one-off signature addition:
How to add custom signature for cpanel clamav - cPanel - Admin-Ahead Community
so there's several ways to about customizing that.