Unfortunarely, the sh404SEF params inside K2 do not offer the option you want. There are 3 category prefix combinations but not one for the root category only. If sh404SEF does not allow overriding this behaviour, you could contact a developer to do this for you.

Since we currently don't offer commercial support services for K2, if you use the contact form here in this website, we can send you back some suggestions for companies that could help you out.