Can i turn my live single site wordpress site into a multisite with sub domains

Hi Guys,

I have a live wordpress site http://www.carcaremagazine.com , we are now looking to turn it into a multi site, to enable our visitors to have their own blogs.

As I understand it I cannot now turn it into a multisite with sub domains, instead of with subdirectories, could someone confirm if this is the case.

If this is the case, am I right in thinking that a multisite with sub directories is not as good seo wise with the search engines for my main site.

Thanks In Advance Andrew @ Car Care Magazine