subdirectory multisite install after 30 days

Hi,
I have this problem: i want to switch to a multisite installation to better organize the contents but unfortunately, wordpress was installed 3 years ago and now it only lets me switch to subdomains such as site1.mysite.com.

For many reasons i would like a subdirectory multisite as http://www.mysite.com/site1

Is there anyway to do it without starting the site over?

Thanks for your help.

Gabriele