How would I override a core block's form_setup_html.php in a package?

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I don't see much point in adding an entirely new image_slider package to the site I'm working on. The core one has mostly everything I need, I just want to extend it lightly. I mean, very very lightly and add some custom templates.

There's a couple radio buttons at the top of the form_setup_html.php for setting arrows or bullets as the nav style. I really just want to add 2 or 3 more values to that for some more nav options, which I'll then create a new default view.php override as well as some new custom templates to use within a packaged theme.

If I create my overrides under applications/blocks then it works just fine but I'd really rather put it under the packaged theme I'm working on. However if I put those same files under my package/blocks it's just not being seen. So I'm looking for recommendations here on the best and correct practice for this override.

In the docs and forums I'm finding various references to overriding/extending other things like controllers but I'm not planning on changing anything in the stock controller.php. I've looked and I don't see any reason why I couldn't just add more radio buttons without needing to change anything in the controller or db.xml, it's already set up to save the numerical value from the radio button but obviously I'm missing something here.

Thanks

wildapple