Best and recommended thing to do is create a new page type that acts as a placeholder for your queries and transformations for those objects which are not located within the Kentico database. So when you create a new page type on the second screen (after you enter a name, choose the 2nd option (The page type is only a container without custom fields) and it will create the container for you without any database table and column representation. This will allow you to create custom queries and transformations.
So after you've created your new container page type, create a new transformation as you normally would. In your transformation use the normal syntax
Eval("YourColumnName") which will be returned in that dataset from your SQL Datasource webpart. In your Basic Repeater, select the new container page type you created and select the transformation you just created and you'll be set.