That's not possible. Moreover, it does not matter what the underlying .wordpress.com blog URL is when you are domain mapping.
There is no expiration date on blogs unused or otherwise. If you can make contact on your own with the owner of an unused blog then there is a means of a consenting blog owner to transfer the blog to another username account. http://en.support.wordpress.com/moving-a-blog/#transferring-your-blog-to-another-user-or-account
if you are not successful, you can register another blog with a similar name. http://en.support.wordpress.com/register-a-blog/
Or you can register a domain via a domain mapping upgrade and it will not matter what the underlying .wordpress.com address is.