Sure! What you’re looking for is a .sublime-settings file. To create settings that will apply to all file types, create a “Base File.sublime-settings” file in your package directory (alongside your plugin’s .py file). The user will be able to override the defaults set in that file by adding the settings to the “User/Base File.sublime-settings” file.
You can also specify platform-specific and file-type-specific default values for your settings. For full details, see the Settings page in the Sublime Text 2 wiki.