WordPress MU and nested sites addresses?

I’m creating my first path-based WordPress Multisite Network. Right now it runs a few blogs, such as:

www.mymaindomain.com
www.mymaindomain.com/blog1
www.mymaindomain.com/blog2
www.mymaindomain.com/blog3

I’m wondering is it possible to change my blogs URLs to something like:

www.mymaindomain.com
www.mymaindomain.com/directory/blog1
www.mymaindomain.com/directory/blog2
www.mymaindomain.com/directory/blog3

It’s quite tricky to achieve with .htaccess as I have absolutely no idea how to tell the difference between sites such as www.mymaindomain.com/blog1 and pages like www.mymaindomain.com/contactpage.

Any ideas?

This seems like it could be solved within your permalink settings. Go to the respective sites settings and add it to your permalink structure.