Yes, this is currently possible (though this may not always be the case in the future). The database schemas are currently identical between the two products though some configuration settings are different between the two products and a new license is required.
Some differences I see with a quick look at the database schema are the profile fields are different, some permissions are different, private messages are enabled for Clearspace X, blog pings are enabled for Clearspace X and viewing of user's email addresses is disabled by default in Clearspace X.
If you diff the database script for sql server between the two products you'll get an exact list of changes between the two versions and you can manually run the insert statements to modify the settings for the internal instance to make it an external instance.
A few notes though:
1. You'll of course want to backup the database prior to making any manual change
2. You will be unable to use the same database for running both an internal instance and an external instance. It sounds like this won't be an issue for you however I wanted to make it clear for any other users that may read this thread.