I believe it can solve your problem about Google´s Sitemap.

1) Copy your feed URL
ex: http://mywonderfulblog.wordpress.com/feed/

2) Cut the last “/”:
http://mywondefulblog.wordpress.com/feed

On Google Webmaster Tools:

3) Add Sitemap

4) Choose type > Add General Web Sitemap

5) My Sitemap URL is:

http://mywonderfulblog.wordpress.com/feed

6) Click Add Web Sitemap

That´s it! It works for me.

Obs: Thanks, Engtech.