Yes, you can do this but at the moment, customising the right-click menu is a bit of a dog's breakfast. This is how to do.
Right-click any toolbar and select Customise.
Click on the Toolbars Tab and scroll down to Shortcut Menu and check the box.
This will open a toolbar for Text, Table and Draw.
This is where it becomes complex: the right-click menus are Context Sensitive! This means that the actual menu that opens when you right-click depends on what you right-click on. Just click on one of the Shortcut Toolbar Menus and see how many there are! Once you have identified the contect sensitive menu you want, open it and then select the Commands Tab to find the command or macro that you want to add to that menu. Then just dragging it over to the Context Sensitive Menu you opened and drop the command where you want it to appear on the menu. Vice versa for removing unwanted commands: just drag a command and drop it into an empty area of screen to remove it.
One of the enhancements that has been requested for Office 12 is that the Contect Sensitive menus are more easily identified. Even if they were just numbered it would make life much easier.
To help ID the correct menu(s) you want to add or remove commands, make a note of what commands are currently listed as this will narrow down the task considerably! Fortunately, for SelectAll, I expect that you will only need to add it to the Text, Text menu.
Regards: tf1