You could, but it will require custom coding to change how it loads the items. The portfolio is actually a blog post which is why it’s loading them by date. The other alternative is to create a manual based portfolio in a page instead of using the blog category + post method. You also have the option of using a plugin like this: http://wordpress.org/plugins/my-posts-order/
There might be other plugins that do this as well so best to check them out before deciding.