Both Weyermann and Briess make some smoked malts. You could simply add these as specialty malts in your extract process with steeping if you keep the percentage to <5% of the total amount of extract.
You can get either one of these from several on line suppliers here is a couple links at Northern Brewer:
Wyermann Smoked Malt
Briess Cherrywood Smoked Malt
Something to keep in mind both of these malts have the ability to self convert. So its likely better to plan a mini mash with either of these. In the case of Weyermann smoked malt you can go 100% of the grain bill. The smokey character is not as super as intense as some would have you believe for these two examples, which is why some german Rauch beers use close to 100% of this malt. Not sure if you could get enough intensity by just steeping it. There are however some super smokey malts out there that you would want to use as "steepers" to get some smoked character.