Depending on how the partitions are in relation to each other physically (ie if they are nest to each other on the disk) you may be able to to it with a live cd with gparted.
If you are unsure post the output of fdisk -l you run this in a console typing su first
You can also create the new partition as seperate ext3, and then mount it under /home. Altnernatively you can use bind mounts to splice in some directories in /home, onto the other partition.
For example in /etc/fstab I splice in “/work/cache/wwwoffle” mounting it under /var/spool as /var/spool/wwwoffle.
/work/cache/wwwoffle /var/spool/wwwoffle auto bind 0 0