Use this script for set automatic Default Signatures for all users reading it's LDAP information.
I asume that user have any information in his LDAP attributes.
echo "Please run as ZIMBRA user"
echo -ne "Checking account: $ac \t"
echo "This account have a Signature. Nothing to do."
echo -ne "Setting signature... "
$RUTA/zmprov ma $ac zimbraPrefMailSignatureEnabled TRUE  zimbraPrefMailSignatureStyle internet zimbraPrefMailSignature "$signature"