Custom provider for User Profile sync

Hi,

I'm in the process of planning for a new project using SharePoint 2013. One of the challenges that the client is giving me, is that they have Active Directory, but they only use it for logging in on their computers and to SharePoint. They have a custom made table in a custom database (SQL Server) where they store all the user information (address, phone numbers, etc.). There's a column here that refers to the user's ID in Active Directory.

Now, as far as I understand, it is possible to sync data into User Profiles in SharePoint from other sources than Active Directory. But I'll have to write some sort of provider to handle this. Right?

I want to point out that the users will log in to SharePoint using their Active Directory user, but when they f.ex. go to their My Site, properties like Address, Phone Number etc. will come from the custom database.

Can someone point me in the right direction?

Thanks!