It's probably easiest to do by booting from a Live CD, i.e. boot from the install and select to test Ubuntu.
Once booted, open a terminal up and create new partitions for /usr and for /opt, although you may not have much in there. These will need to be logical partitions soyou'll need to create an extended partition (/dev/ada4) first.
You'll need several temporary mount points, so I usually create subdirectories in /mnt (e.g. /mnt/root, /mnt/usr and /mnt/opt) copy the existing contents across, then edit the existing fstab (which will be in /mnt/root/etc/fstab) to mount the new /usr and /opt.
Sequence of commands is (UNTESTED!!!) in Live CD terminal is roughly:
# fdisk /dev/sda
.... create new partitions. 4 (extended) 5 (to hold /usr) 6 (to hold /opt)
# mkdir /mnt/root
# mkdir /mnt/usr
# mkdir /mnt/opt
# mount /dev/sda1 /mnt/root
# mount /dev/sda5 /mnt/usr
# mount /dev/sda6 /mnt/opt
.... copy data
# cp -dpRxv /mnt/root/usr/* /mnt/usr
# cp -dpRxv /mnt/root/opt/* /mnt/opt
.... edit /mnt/root/etc/fstab
.... i.e. add extra lines to mount /dev/sda5 on /usr and /dev/sda6 on /opt
At this point, you you be able to boot from /dev/sda and check everything works OK, but you will notice that we haven't deleted the old contents of /usr and /opt from /dev/sda1; we've just mounted the new directories over the old ones and hence hidden their contents. This is so if it doesn't work, you can just undo the edits from fstab (delete or comment out the extra lines you added) and you'll be back where you started without losing anything.
Finally, when you've check it's OK, boot into the Live CD again, mount /dev/sda1 on /mnt and remove the old contents of /usr and /opt
# rm -Rvf /mnt/usr/* /mnt/opt/*
While I have restructured disks in the past, there is always the danger of mistyping a command and wiping out some data. In this case, since you never mount /dev/sda3 you should be OK, BUT I OFFER NO GUARANTEE AND YOU SHOULD CHECK CAREFULLY THAT YOU UNDERSTAND EACH STEP BEFORE TRYING ANYTHING!