I have a road bike with look style pedals, and I am looking for an attachment I can clip-in to the pedals (where my cleats clip in) to provide a platform so that I can temporarily use ordinary shoes to pedal. I find this convenient if I have to lock up/walk somewhere after so I can wear whatever walking shoes I want (I can't walk in my look cleats and don't want to convert to SPD).
I don't know of any commercial attachment, but there are at least two other ways you can solve the problem.
Mikes linked to a DIY solution for eggbeaters. You could easily do the same with Look-style clipless pedals. Just buy a spare pair of cleats for and bolt them to a piece of wood or plastic. You'd need to drill three holes in the flat support piece, but you can probably buy all the hardware (including cleats) for under $30. The disadvantage of this method is that it would be difficult to properly weight the clip-ons so that they rest in a convenient orientation when you foot is off the pedal. You may have noticed that Look pedals always sit with the front pointed upward. This done intentionally so that you can simply slide your shoe and cleat forward into the pedal to clip in.
I know that the question asks for something to clip on to the pedal, but it seems like the real problem you're trying to solve is "How can I walk around in the same shoes I ride in?" For that, I would propose an alternate solution: cleat covers! These are rubber covers which fit over the cleat, available at just about any bike shop in a variety of generics and brand names.
You can, of course, also use the pedals as-is with your street shoes. For "short" distances (every rider is going to disagree on what short means in this context) I find this works acceptably well. I place my feet on the pedals the same as if I were wearing cycling shoes with cleats. In other words, I use the top of the pedal and place the ball of my foot over the axle.
Walking around in cycling shoes with cleat covers can be a little awkward at first because your toes are pointed slightly upward (the covers have to go over the cleat, obviously, so that adds even more thickness to the front of the shoe). Most people get used to them pretty quickly, though, and it's really not a big deal. One thing you can try is walking on the balls of your feet. Normally that wouldn't work in cycling shoes because the cleat is smooth and slippery, but with cleat covers you get just as much traction as you would in regular shoes.
Personally, I wouldn't do it. For the platform to be of any benefit it will have to have grip of some sort. The resulting grip combined with the torque from your foot twisting naturally can cause the platform to un-clip accidentally.
I would recommend just buying a cheap pair of platform pedals and swap as required.
http://www.cyclelicio.us/2012/diy-flat-pedal-adapters/ If you have looked everywhere and cannot find anything this might work for you
Fly Pedals came out with the best version of a clipless pedal adapter. Works with every kind of clipless pedal, and the best part is they have a lifetime warranty. They sell on flypedals.com but shipping is cheaper on Amazon
There is the Deckster clipless pedal adapter. Works with most 2-bolt cleats.