How to load another model in beforeSave() function ?
If it is an associated model you can access it as normal. Otherwise you break MVC if you want to load an other model.
As pointed out, it breaks MVC, but if you really need to without associations, use the TableRegistry:
http://book.cakephp.org/3.0/en/orm/table-objects.html#getting-instances-of-a-table-class