This is very specific to what need to be done when the user clic on that button, also the "Add an Item" link is related to One2many and Many2many fields in a form, and there should be a way to restrict when the button is needed because if not it will be applying for all those field widgets that get the extension.

Let's show how could be done as a draft

openerp.your_module = function (instance) {
    instance.web.form.One2ManyList.include({
        pad_table_to: function (count){
            var self = this;
            var res = this._super(count);
            this.$current.find('.oe_form_field_one2many_list_row_add').append(
                $('<a>', {href: '#'}).text(_t("Filter item"))
                    .mousedown(function () {
                        if (self.view.editor.is_editing()) {
                            self.view.__ignore_blur = true;
                        }
                    })
                    .click(function (e) {
                        e.preventDefault();
                        e.stopPropagation();
                        if (self.view.editor.form.__blur_timeout) {
                            clearTimeout(self.view.editor.form.__blur_timeout);
                            self.view.editor.form.__blur_timeout = false;
                        }
                        self.view.ensure_saved().done(function () {
                            //run your js code here
                        });
                    })
                );
            }
    })
};

To do it for the Many2many fields just change:

instance.web.form.One2ManyList              by      instance.web.form.Many2ManyList

.oe_form_field_one2many_list_row_add    by      .oe_form_field_many2many_list_row_add

Also you will need to change your_module with your module name and add this js file to be loaded by the assets in the backend in an xml file like:

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <template id="assets_backend" name="my js assets" inherit_id="web.assets_backend">
            <xpath expr="." position="inside">
                <script type="text/javascript" src="/your_module/static/src/js/file.js"></script>
            </xpath>
        </template>
    </data>
</openerp>

This put the link as clickable to run some js code but I think that this is just the peak of the iceberg but perhaps could help you and others