Manual Subdomains with WordPress MU

I’m trying to set up a series of blogs for my family. I’d like to have each person’s blog be firstname.domain.com. I have WordPress MU loaded on the server, but I don’t have access to edit the DNS records. I do have a decent knowledge of PHP, so I was wondering if I would be able to set it to use sub-directories (on the install page), then manually modify the files to use sub-domains.

I am new to WordPress, so I apologize if this is a stupid question.