I have 1 DSL Router and 2 Airport Extreme. I want to go to each Airport with Lan from my Router. But i want only 1 WiFi net. Is this possible?

Sure. Apple's setup "wizard" will set things up this way by automatically, if you set up the first Extreme connected your router and then temporarily connect the 2nd Extreme to the first Extreme to set up the second Extreme.

Once the 2nd Extreme has the correct settings, then you can connect it directly to your router.