The ManageBibliographicSourcesDialog which you wish to modify is implemented as a separate window. For customization purposes we've exposed an extensibility point that allows you to change the dialog altogether. If you look at the attached demo you may notice that the MyManageBibliographicSourcesDialog in it is marked with the following attribute:
What this means is that every time the ShowManageBibliographicSourcesDialogCommand is invoked, which is what happens when the Manage Sources button on the ribbon is clicked (refer to attached image), the custom implementation will be invoked.
As for the particular demo, I did not actually modify the dialog. I merely used its default implementation to show how you can change the dialog with a custom one. If you want to modify it you should implement the relevant code in the MyManageBibliographicSourcesDialog.
I hope this is helpful!
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