Since I've modified Twenty Ten without creating a child theme, can I somehow copy what I've got?
yes, you can make a new sub-folder in the themes folder (for instance:
/wp-content/themes/myowntheme ) and copy the full content of the /twentyten folder into it.
then edit style.css in the new folder and change the name of the theme:
(create your own inspired name for your theme)
/*
Theme Name: My Own Theme
Theme URI: http://wordpress.org/
Description: a theme modified from The 2010 theme for WordPress
Author: the WordPress team, mod wendymello
Version: 1.1
Tags: black, blue, white, two-columns, fixed-width, custom-header, custom-background, threaded-comments, sticky-post, translation-ready, microformats, rtl-language-support, editor-style
*/
that is all.
you have to switch over to your 'new' theme in 'dashboard' 'appearance' 'themes'
---
the differnce to a child theme is, that your own copied theme will not get the latest updates which the parent theme might get.