You could do it the other way around just to make it easier. Make normal collision polygons for all objects - so you can interact with them normally, like hover mouse over them, etc.
But for proper collisions with players you could use separate object. like Burvey suggested. Make a 2x2 sprite or tiled background and simply copy it over and placed on the layout making kind of a "level collision map".
Also if you want to make player go behind or in front of the objects you can do that with just a Origin or Image point of the sprite:
Player.Y > Sprite.Y = player is in front
Player.Y < Sprite.Y = player is behind
....but it all depends on what kind of style your adventure game have.