To programmatically add to menus you can do as described here:
What can I do to programmatically add to contextual menus? Is something similar to
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There's an option
ContextMenu, which you seem to be able to set at cell, notebook, or even box level. They all default to something like
That info seems to be in the ContextMenus.tr file in the
$InstallationDirectory, SystemFiles, FrontEnd, TextResources.
In any case, all that that option needs is a list of
MenuItem[...]. You can look at that file (or at MenuSetup.tr) to get a hang on the syntax. Then it's just setting the option.
Apart from the brute force method of reading the file, creating your own, adding your Menus, and setting that option, I still haven't found a way to "append" the MenuItems. Only to replace... I'll edit if I find something