It can be done with custom modification. You have 2 possibilities:
1) If you want that the button is displayed only on a special page (by yourself created page) you have to put in the code you get from this site:
http://www.google.com/webmasters/+1/button/
2) If you want that the button is displayed on every page just install the
Advertisement plugin. Then add a banner. You get the code from this site:
http://www.google.com/webmasters/+1/button/
At last, edit the "Banner placement".
That's all :) Very simple!