You cannot do this, Serendipity does not offer the concept of "sub-blogs". We only offer "sub-categories" and "sub-authors", but they all direct into one single blog.
With the permission system though you can create users that only have write access to specific categories. With the plugin "Extended properties for categories" you can choose a different template for each category, which might come close to what you want.
The last, but hard way, is to setup a shared installation like outlined on
www.s9y.org. You can then write a script that automates creating a shared installation, but for that you need a rather good PHP/Apache knowledge.
Regards,
Garvin