I've never done it but with a bottle carbonated beer like Sierra Nevada you can carefully pour the beer out of the bottle, saving the last little bit of beer in the bottle which contains the yeast. Do this to 3 or 4 bottles, then make a yeast starter (look this up if you dont know how) and add the yeast from the bottles. You might have to step up the yeast starter a few times to get enough yeast for pitching, but it should work. Good luck!