Tis not possible. 10.1 never came out for ATV2 (the code being used actually jumped that version and went to pre-11 rather than merging the ATV2-branched code with the 10.1 code).
The closest thing possible is "10.0-7", which is the last version with dharma compatible code, but is full of bugs and is a massive step backwards.
There are a few things you can do and not have to downgrade:
•Check out the add-on sub-forum and check for new/test versions of the add-ons you want to install.
•Force XBMC to use existing add-ons (sometimes works, sometime doesn't) by doing this:
http://forum.xbmc.org/showpost.php?p=823821&postcount=2
•Contact the authors of your missing add-ons and tell them to get their butts in gear and update either their code or check for compatibility so the add-on can be added back to the official repos.
Can you give us a list of which add-ons you are looking for? Just today I helped someone find some alternatives for their add-ons, and the end result was that we actually found something better than what was in the official repos.