Not an official answer. I hope it can help anyway :-)
As far as I know it is not possible to customize the property editor section (bottom right).
An extension can however create a "panel" (many extensions do that)
that goes just under the diagram.
The Rebel extension for instance create a panel with specific purpose
properties and information used for the generator. It somehow replace/complement
the staruml hardwired property editor, which is still there but not necessary used a lot
(this depends on the extension).
The "markdown" extension is another example. It allows to edit the "description"
field with markdown rendering. So instead of changing the hardwired property
editor, the idea is to create the customized behavior in a panel.
You can also use dialogs but this is probably not what you want.
So my answer is NO you cannot modify property editor,
but YES you can create a panel on your own and do whatever you want :-)