I guess you mean that you want to _add_ a partition to the existing partitions on both your internal, and an external USB drive. Do I have that right?
Of course, that's one of the enhancements to the Disk Utility in Leopard 10.5
You can add a partition, as long as you don't try to make a partition that will lap over onto the existing files that you have.
Just open Disk Utility, select your drive (not its partition) and go to the Partition tab. Click the plus (+) at the bottom of the partition list. Slide the partition slider to change the space for that new partition.
Just don't forget to leave enough space for your existing partition. Your files, plus about 10GB should be OK. The rest of the drive could be used for that partition....
This will work, but like any partition scheme, there will be some risk to your data - make sure your files/documents are backed up safely before you try this.
Serendipity is a lucky guess !