I'm not sure about launcher pro, but you can do this fine in the stock infuse touchwiz ui.
bring up applications (from applications button).
hit menu, change "view type" to "customizable grid".
then hit menu again. this time there will be a second menu option, "edit". select edit.
all buttons, including the ones in the bottom menu dock, will now have squares around them.
Simply drag whichever icon you want over one of the ones there and let go. The one that was there will move to occupy the spot your other icon was (which may well be out of order if they were in order). If you keep customizable you can rearrange them how you like, or just put them back in alphabetical. Either way what you swapped in the dock will stay in the dock.
If yo do this sans launcher pro, then install launcher pro, will it pick up on what you swapped out?