You seem to have found your way to fdisk. Keep in mind that its editing features have no real safeguards against accidentally using erase (which removes all the partitions) or editing a partition that Mac OS X is currently using, and also few safeguards against even entering numbers that don't make any sense, so be careful.
You can use fdisk in interactive mode to edit the entries in the partition table:
sudo fdisk -e /dev/rdisk1
Then edit your first blank partition table entry (e.g. edit 3), set it to type AF, press return for the default to the CHS mode question, press return to accept the default of starting the partition after the end of the last one, then enter the end sector number you want (the default is the end of the disk; to figure out one for a size you want: divide the size you want by 512 bytes to get the number of sectors you want, and then add that to the start sector number). Do a print to make sure it looks okay (nothing has been saved yet, so if there's a mistake you can just exit (not quit) at this point and try fdisk again.) Then write to save the partition table over the old one on the disk.
If you get a warning that the changes will require a reboot, that's okay; answer y, and then reboot Mac OS X.
Now the partition table has been updated, but it still has the old contents; Run Disk Utility, and from the Erase tab, choose the new partition, give it a name, and Erase it.