Hi dake1,
I'd say there's no need to migrate (in terms of moving data to sharepoint's db). What you can do is, use SharePoint's content db as it is and also have your db side by side in the same SQL instance and have your application configured to use both depending
upon task at hand.
But this again depends on what part of your CMS you want sharepoint to handle (an example would be Search) in which case some of your modules will be obsolete/not used and you have to very carefully plan this out.
A random burst of technology notes Blog:
http://rhythmiccoding.blogspot.com