I dont think you can do what you are talking about.
Just get your own domain and host it there too. Use hthe rss to blog plugin to get your old posts into a new wordpress install. Or just start another blog on your own domain.
You will take a temporary hit on your
PR but will be worth it in the long term.
Godaddy is has a great easy wordpress install in their hosting connection service.
This is why I always recommend to never, ever use blogger or a free blogging service.