[Django]-Insert more records at once in raw_id_fields

It’s possible. In order to select more than one record, you need to override default dismissRelatedLookupPopup() in django/contrib/admin/static/admin/js/admin/RelatedObjectLookups.js by including your script in the Media class of your ModelAdmin or widget:

var dismissRelatedLookupPopup = (function(prev, $) {
    return function(win, chosenId) {
        var name = windowname_to_id(win.name);
        var elem = document.getElementById(name);

        // 1. you could add extra condition checking here, for example
        if ($(elem).hasClass('my_raw_id_ext_cls')) { // add this class to the field
        //     ...logic of inserting picked items from the popup page
        } 
        else { // default logic
            prev(win, chosenId);
        }

        // 2. or you could copy the following part from RelatedObjectLookups.js ...
        if (elem.className.indexOf('vManyToManyRawIdAdminField') != -1 && elem.value) {    
            elem.value += ',' + chosenId;
            // 2. and add a return. Remember this acts globally.
            return;
        } else {
            document.getElementById(name).value = chosenId;
        }

        // 3. the following line cause the popup to be closed while one item is picked. 
        // You could comment it out, but this would also affect the behavior of picking FK items.
        win.close(); 

    }
})(dismissRelatedLookupPopup, django.jQuery);

Django does not support this by default. There are some snippets at djangosnippets.org, you may want to take a look at them.