Yes, you can import your existing user database. You can either do this by writing a script and have that execute on your web site or by directly manipulating the underlying SQL table / XML file (depending on what you use to store Composite C1 data). You can also build a provider that links your existing user database with Composite C1 Extranet.
Importing users programmatically: For a script approach please see methods like AddNewUser described on http://docs.composite.net/Packages/Community/Extranet/CompositeCommunityExtranetDeveloperGuide/Using-Extranet-Facade-Methods
You would write this script as web service, aspx page or similar which executes on the Composite C1 website.
If you are running the Extranet in a default setup expect the providerName
to be "Default".
Manipulating the physical data store directly: This depends on what data store you are running on. I suggest you add the groups you want and a test user to help you recognize data when you look at the underlying XML files / SQL tables.
If you are running on XML (default) you should focus on the files named Composite.Community.Extranet.DefaultProvider.DataTypes.DefaultProvider*.xml located in the folder ~/App_Data/Composite/DataStores. There are 3 sush files, one for groups, one for users and one for the relation between users and groups.
If you are running on SQL Server you should focus on the 3 tables named Composite.Community.Extranet.DefaultProvider.DataTypes.DefaultProvider*
In both cases you would need to add new entries to the User table/xml file and matching group relations to the GroupUser table/xml file. When you add a user you provide a unique ID and this ID you reuse to register the user in GroupUser.
When you have made your changes you can force Composite C1 to reload by using the Tools | Restart Server command in the C1 Console. If you make a backup of files/tables before you make changes you can easily revert by restoring the backup (in case you need to start over).
Writing a user/group provider: If your user data is in an external store and you would like to keep it there you could also make a bridge between this existing user store and the Composite C1 Extranet by creating a custom provider. If this is relevant see http://docs.composite.net/Packages/Community/Extranet/CompositeCommunityExtranetDeveloperGuide/Writing-Custom-Extranet-Providers