Currently, there are no default shortcut keys for adding attributes. You can assign it using Options dialog - Menu tab - search for the action in the right pane (e. g. AddAttribute) and set the shortcut keys in the textbox New in the Shortcut Keys group box below.
Another option, how to edit the class using the keyboard, is to press F2 and the Down key. You can navigate through the attributes. When you reach the last attribute and press the Down key again, it will add a new attribute. To navigate to the operations section, press Ctrl + Down.