Hi – in our current release you can do this by means of a template override. Please see our Themer’s Guide for an overview of the basic process, but basically you would be interested in customizing the community/modules/taxonomy.php template. Within the heart of that file you can see this line of code:
<?php TribeCommunityEvents::instance()->formCategoryDropdown( null, $currently_selected_category_ids ); ?>
Despite the name it actually produces checkboxes rather than a dropdown – but in any case you could replace this with your own custom code to generate a list of categories within a select element (a dropdown).
Does that help here?