I'm not sure what you mean by over a base installation.
You can map a domain like that. If you already have a WP install at said domain and you change the DNS to point to your multisite install the original base installation will no longer be accessible.
I have previously blog with wordpress.com as http://basunivesh.wordpress.com now I purchased domain and hosting from other party. But using WordPress.org and it's themes. I need help like how my old blog re-direct to my new blog
@Basavaraj - ask the folks at the wordpress.com support forums. The redirect has to be done at wordpress.com.
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