You can add malware expert ClamAV signatures to freshclam.conf file:
Also replace serial_key_here your subscription key (ex. 21r3jx79pcm9tj50z5lnuannp), if you dont have Buy subscription.
DatabaseCustomURL http://signatures.malware.expert/serial_key_here/malware.expert.ndb DatabaseCustomURL http://signatures.malware.expert/serial_key_here/malware.expert.hdb DatabaseCustomURL http://signatures.malware.expert/serial_key_here/malware.expert.hsb DatabaseCustomURL http://signatures.malware.expert/serial_key_here/malware.expert.ldb DatabaseCustomURL http://signatures.malware.expert/serial_key_here/malware.expert.yara DatabaseCustomURL http://signatures.malware.expert/serial_key_here/malware.expert.fp
Your buyed subscription key found in here!