Having subfolder structure (in URL) for MultiSites

Hi,

I have a wordpress installation and have just enabled the multi-site functionality.

What I am trying to do is to have the multi-sites run under a different folder (slug).

Ex:- my primary wordpress installation is on http://www.example.com
now I want the multi-sites to be exist as:-
http://www.example.com/sites/site-1
http://www.example.com/sites/site-2
http://www.example.com/sites/site-3
..
.

How can I have a /sites/ (or any other word) prefixed in the slug? Changing the permalink structure on the primary site (adding /sites to the Custom Permalink structure) will actually make this work, but problematically, it would result in addition of the /sites/ word for every post on the primary site too!

Looking forward to a solution

Thanks,
Vik