DNN is way more than just a front end to a database.  If you are needing to use the database and tables you have already you are pretty much out of luck without a whole lot of work.  So much so that it would make since to use DNN.

You can use DNN with your current DB though.  When you install it will add all the tables DNN uses to your current DB.  Just change the connection string to point to the name of the DB you want it to use.

One problem you could run into is if you have a table name that DNN wants to use.  In that case I'm not sure what exactly would happen. Either DNN would drop your current table and create a new one with the schema it expects or not create the table and fail the install. 

Even if everything works correctly you will need to write modules to take advantage of the tables currently in your DB.

All in all you would probably be better off just creating a new DB for DNN and porting the information you need from the current DB.

Cheers,