You can do it after the fact. Here is what I did to expand my /UNIONFS in knoppix. I mounted a free partition at /mnt/sda1, then did a "unionctl /UNIONFS --list" to make sure I know the top priority part of the union, then a --add to stick the new partition on top. df shows I now have plenty of elbow room in UNIONFS. Whether this is persistent or not is another question.

mount bla bla bla

df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/root 2471 19 2452 1% /
/ramdisk 808568 662484 146084 82% /ramdisk
/UNIONFS 2750524 2604440 146084 95% /UNIONFS
/dev/sda2 1035692 741788 241292 76% /cdrom
/dev/cloop 1941958 1941958 0 100% /KNOPPIX
/UNIONFS/dev/sda4 150689348 25805684 117229096 19% /homey
/UNIONFS/dev/sda1 1035660 33176 949876 4% /mnt/sda1


unionctl /UNIONFS --list
/ramdisk (rw)
/KNOPPIX (r-)


unionctl /UNIONFS --add --before /ramdisk --mode rw /mnt/sda1

df -h
Filesystem Size Used Avail Use% Mounted on
/dev/root 2.5M 19K 2.4M 1% /
/ramdisk 790M 647M 143M 82% /ramdisk
/UNIONFS 3.7G 2.6G 1.1G 71% /UNIONFS
/dev/sda2 1012M 725M 236M 76% /cdrom
/dev/cloop 1.9G 1.9G 0 100% /KNOPPIX
/UNIONFS/dev/sda4 144G 25G 112G 19% /homey
/UNIONFS/dev/sda1 1012M 33M 928M 4% /mnt/sda1

web pages I found helpful:
http://www.unionfs.org/
http://www.fsl.cs.sunysb.edu/project-unionfs.html
http://www.knoppix.net/wiki/Trivial_...ion_filesystem

this may also be of interest - combine unionfs & gmailfs for true mobility:
http://www.knoppix.net/wiki/Gmailfs