It's not something I want to introduce into the plugin, but you could certainly edit your copy to work that way. Are you familiar with WP_Query?
Edit your file and change wp_dropdown_categories() on line 92 to a plain text input field where you can enter category IDs separated by commas. (Or, if you want to get fancy, write a loop that prints a checkbox for each category, then edit line 64 -- $instance['cat'] = $new_instance['cat']; to implode the resulting array into a comma-separated list.)
And change its name so you don't get notified about automatic updates, which would overwrite your changes.