You may be able to do that. However, many BMX seats have larger diameter seat rails that may not work with a roadbike seatpost well.
Also, keep in mind many BMX seats are not designed for sitting on for any distance/long length of time. They tend to not be the most comfortable.
What wds178 said about comfort is true for some seats, but I've had a bunch of different BMX seats that were plenty comfortable for long distances. Definitely make sure the rails are the proper size though.