If I read your post correctly, then what you have listed as 'subsite'
below is actually a site  collection. In that case, yes, it is easy to
do.

1. Backup your Farm

2. From the 12 hive run: stsadm -o backup -url
http://server/sites/subsite -filename c:\subsite.bak

3. then, stsadm.exe -o restore -url http://server -filename
c:\subsite.bak

If you only have one content database for the http://server web app, you
might get an error due to a Site Collection ID collision. In that case,
add a second content DB to the Web app.

You read my post correct, and I was finally able to get it to migrate.
I had a heck of a time getting all the permissions configured correctly
so that it would run.