*** update that works directly without more investigation, i will left the original answer as a reference ***

Odoo class/widget functions extend and include have something in common. That is that both functions apply the received javascript object properties to the class/widget. The difference between those Odoo class/widget functions is that the extend function is used to get a copy of the Odoo class/widget with the modifications without been modify the original class/widget by returning the modifications in a new class/widget. The include function will do the modifications in place to the class/widget without returning anything new.

To reach the class/widget Many2ManyListView for been able to use extend or include functions the best way is using the following code:

var core = require('web.core');
var FieldMany2Many = core.form_widget_registry.get('many2many')
var extention_not_done = false;
FieldMany2Many.include({
    init: function() {
        this._super.apply(this, arguments);
        //if(extention_not_done){
        if(!extention_not_done){
            var Many2ManyListView = this.x2many_views.list;
            
            // use include to modify the original
            Many2ManyListView.include({
            
            });
            
            // or use extend to create a copy with the modifications
            var Many2ManyListView_New = Many2ManyListView.extend({
                
            });
            extention_not_done = true;
        }        
    }
});

That technique consist of using an include extension to the FieldMany2Many init and call the original init function by calling the super so the class widget will be initialized and you could then access to the original Many2ManyListView class/widget. Just need a check due the init of the FieldMany2Many will be called by many times and we don't wanna applying the extension to the Many2ManyListView several times, we just need one.

*** original answer ***

Try it this way:

var core = require('web.core');
var FieldMany2Many = core.form_widget_registry.get('many2many')
var Many2ManyListView = new FieldMany2Many(args).x2many_views.list
/*** examples ***/
Many2ManyListView.include({
});
var Many2ManyListView_New = Many2ManyListView.extend({
});

With tha code you just need to investigate what need to be passed as args for the FieldMany2Many instantiation and you will be able to call the include or extend on the retrieved Many2ManyListView