Wildcard domains as main domain?


I have asked this once, but this is somehow different question.

If I use some domains as multi-domains (where users can choose sub-domain for their blogs), I know that I can't build a site on main domain. For example, my users can choose sub-domains - domain1.com domain2.com and I know that I can't build site on domain1.com or on domain2.com..

I also know that when visitor enters domain1.com or domain2.com it redirects to main blog, but can it be redirected to for example site.domain1.com, site.domain2.com.

Something with .htaccess or.., anyway - is it possible or not?