By Design, there is one permalink structure for the whole wordpress site. What you try to achieve needs to introduce a custom permalink structure for a part of it.
This is not trivial because you need to introduce your own permalinks via rewrites (Rewrite API (Wordpress Codex)) and you need to change the links to those pages when linked on occasion. This can be done with hooks for the appropriate functions. E.g. for get_category_link() you can make use of the
I haven't done this yet for a larger part of a site and I assume that it can introduce problems as you need to take care of multiple places to have it working properly. I don't want to say it's not possible but just a reminder of the side-effects this can have.