If I understand you correctly, you are wanting to pass another disk device to your DomU. I have done this a few times in Ubuntu 8.04.
In my DomU config file, I add another device line such as:
disk = [
Restart the DomU and then within the DomU virtual machine, edit the fstab file.
# /etc/fstab: static file system information.
# <file system> <mount point> <type> <options> <dump> <pass>
proc /proc proc defaults 0 0
/dev/xvda1 none swap sw 0 0
/dev/xvda2 / ext3 noatime,nodiratime,errors=remount-ro 0 1
/dev/xvda3 /home/files ntfs-3g defaults 0 0
In my case, the third entry is the extra device we passed and is an NTFS partition for my file server.
Then just issue a "mount -a" command at the command prompt and you should see your new partition mounted where you specified.