If it is crossthreaded it may have damaged the insert as well. Impact screwdrivers can work for removing seized screws just make sure the underside is well supported or you may drive the insert through the board. Dont hit it to hard. If you are removing a set of foot pads to replace them and discarding the old ones you can cut the foot pad off around the screw and use a pair of pliers multigrips or vice grips to grab the screw. a dremal tool with a cut off disk would work well. You can also use a screw driver with a hex on the end that you and put a spanner on to get a bit nore leverage try moving it back and forwards to break away whatever is holding it. There are also some sprays you can try if you can get them near the threads like rost off(worth), CRC, WD40 etc. I also used a spray recently that freezes the screw by spraying the head, this shrinks the screw and should make it come out imediatly after. cant remember what it is called but got it from balckwoods. Good luck