IBindingList implements IEnumerable; therefore, as long as you are returning an IBindingList and its AllowNew property returns true you should have no issues inserting items in details.

Take a look at the Creating a Custom Detail Description example in the documentation. In the GetDetailsForParentItem, I return a new BindingList as such (using the GetNewBindingList method):

       object details = foundProperty.GetValue( parentItem, null );
       Type detailsType = details.GetType();
       MethodInfo getNewBindingList = detailsType.GetMethod( "GetNewBindingList" );

       return ( IEnumerable )getNewBindingList.Invoke( details, null );

Even if it is cast as an IEnumerable, I am still able to insert new data items.