Up to version 10.5.4 of Symantec Messaging Gateway it is not possible to define sender-based exclusions for Disarm Content (Malware Policies).
Using Policy Groups only internal recipient groups can be handled differently for various policies.
Please implement filter conditions (as it is possible for Content Filtering Policies) for Malware Policies to define exclusions based on various conditions (eg. sender-address / sender domain). Another possibility would be to integrate this into Local Good Sender Domains (only bypassing of Content Filters is possible there).