No. You can't fit an oval object into a round hole. Although, most aftermarket 6x9s should offer below deck mounting. If there is room below the deck, you can drill new holes for mounting and use the factory grilles. If not, you would have to cut 6x9 holes in your rear deck, again if there is enough room.
6.5 subs are a direct replacement. I put elemental design free air subs mounted on the bottom side. Magnets were to big to drop through the opening as it is not round and I did not want to cut the deck.
You can pop the rear speaker covers off from underneath. Go to the outside corner of the speaker from the trunk and push up with a 1/2 dia wooden dowel or similar. Pops up then work around till you can get it out. This way you do not mar the top trying to wedge out with a small screwdriver.