Custom Context Menus For Any WB (using Add-in?)

Ok, so I have this neat little macro that on right-click brings up a custom context menu that displays a few different options - for example one such function is simply copying the selected cell content to a (specific) different sheet. It is a very useful feature in the context I am normally using it to merge information from different locations in a new structure (well, I think this is irrelevant, but just letting you know basically what I am doing).

Anyhow, to implement the custom context menu, I am using events in the ThisWorkbook object, namely Activate to add context menu entries and DeActivate to remove them if switching workbook.

Now, if I receive any workbook from someone else, I'd like to add this feature to get the custom context menu easily without having to copy the received workbook into a new workbook (it's a bit of a hassle, and ideally, if the received workbook has any other macros, this function could just be merged). I was thinking using an add-in to achieve this, but using the ThisWorkbook events in the add-in doesn't trigger the events (I guess because the hidden workbook in the add-in is never "Activated" or "DeActivated").