There is no simple way to identify how an item was selected, and there is no function or facility to do so. You may be able to detect how something was selected by placing the list box in an alternate state and using set analysis to combine direct selections and selections in the alternate state.
But even if you could do this, you would not change the behaviour of the drill down dimension. This responds to the possible values that arise from the selections, regardless of where and how those selections are made.
One way around this, using the alternate states mentioned above, is to make two conditional dimensions. the dimensions would be selectively displayed, based on whether a selection is made to the alternates state, so the table can switch between a drill down and a normal field dimension. But it may be complex to achieve seamless and intuitive behaviour and you may simply confuse your users.
Logic will get you from a to b. Imagination will take you everywhere. - A Einstein