Let me first try to rephrase your question based  on what I understood:
"Can I create a root site  from Blog temaplate and have multiple subsites under
this root  site?"
Assuming I could understand correctly, my answer is YES. My logic is that you
can create root site from any template  and have subsites under it.

I guess that I did not make my question  clear. I am
talking about after fact, not before fact. I know you can create a blog site  as
the root  site, then create many subsites based  on different tempaltes. But I am
not sure whether you can convert/change/modify an existing root site created
based on publishing  template to an site based on blog  template or an site with
blogging functionality, also please note in the site collection  there are many
sub sites  already.

I don't think above could be done easily, if not impossible.

You can't change the site  definition for an existing site once its been created.
You can however activate any features that are automatically activated by the
site definition to achieve a similar result. However, I don't know if its just
Features that supply most of the functionality for a BLOG site or whether there
are custom lists, etc. built  into the site definition. I suspect its the
latter.

Have you tried to go to your root  site, then site  Actions>>Site Settings>>Page
Layout and Site Templates Settings and then add the blog  site template? This
works in a publishing  site, but I don't know about team sites. I have created a
root site of publishing but then added the wiki and blog site templates for
future projects that I'll be working on. I haven't done much more than that
though with the Blog or Wiki sites.

This makes it possible to create subsites that are blogs or wiki's, but it
doesn't change the nature of the existing site. He wants to change the root
site from being based  on a Publishing definition to one based on a blog  site
template.

I think it is fair to conclude that a publishing  site can't be converted to a
blog site. What should have done at the begining is to create a blog site  as the
root site, then enable publishing features. Although I have not tried above.

At the end, I used SPD and dataview web part to surface the blog  posts to the
home page of the publishing site and it looks pretty good, at least from ui
perspective.