Multi site clarification

Here is what I think you need to do

Create a NEW multi site blog on the new “master” domain with WP 3.0

Create a new empty blog the the url h..p://www.mysite.com/oldblogname

Export you old site and input inport into the new blog

You need to check that all the images get copied to the new site OK otherwise keep the old blog in place to serve the images

And you will have I nice new blog

To keep it tidy you should put a 304 redirect from the old URL to the new URL

Something like this should (not tested) into a .htacces file in the old blog folder

RewriteEngine on
#
RewriteRule ^(.*)$ http://www.websiteA.com/oldblogname/ $1 [R=301,L]