Definitely don't do db insert directly--Clearspace caches data, and to do this, assumes that no other application is creating/modifying the data in the database.
You really have two options:
Create a custom SSO to use your "official" data store for users. If you are using LDAP, Clearspace provides integration out of the box. If it is to some other user data store, then you can create your own SSO implementation as outlined in the documentation.
Use Web Services.
Of these two options, I strongly recommend option #1.
Hope this helps!t
Ok thanks for the input, that definitely seems like a possibility.