You can add fields into the mass update by enabling it from studio.
For example:
Navigate to Admin > Studio > Contacts >Fields
and edit the fields that you wants to and mark check box “Massupdate” true and resave the field. you will have it that field in mass update.
In case if it doesn’t work for specific field. You can do it manually.
custom/Extension/modules/{module}/Ext/Vardefs/sugarfield_yourfieldname_c.php
and
$dictionary['YourModule']['fields']['yourfieldname_c']['massupdate']=true;
Do quick repair & rebuild.