The easier way - create a new disc image and use bsd.rd to partition, label and format it as needed. When done with it, mount all of Your volumes and use tar or cp to populate the new image. A side effect of this approach is that You can test the new image before loosing the old one.
You could also use disklabel and growfs, but this way You would still want to backup Your data, so just using a new image is somewhat cleaner.
