Hi,

If you need to call an existing action from an Odoo Module inside your custom module, you can do it with the External ID of a record. 

Suppose if you take an example, You need to call the action of All Transfers menu inside your custom module, first of what you have to do is that, you have to find the external id of that action. This can be done from back end by checking the module name and the record id.

 If you want to get it from UI, activate the developer mode and go to Settings -> Technical -> User Interface -> Menu Items, search and open your menu and click on the action field, then you will be redirected to the corresponding action. Then you can get the external ID of the action by clicking the debugger button near in the logged in users name in menu bar, Click View Meta Data, then in the pop up you can see XML ID, ie the External ID.

So the external ID of the menu All Transfers is stock.action_picking_tree_all, so you can add it your menu ,

<menuitem id="test_menu"
          name="All Test" action="stock.action_picking_tree_all"
          parent="test_root"
          sequence="10"/>

Thanks