LVM is a great tool for volume management. You can easily add a new (virtual) harddisk to a existing logical volume.
With fdisk -l you can see what devices are available. Choose the right one and execute:

fdisk /dev/sdb

Create a new partition which fills up all the free space with the type Linux LVM (HEX: 8e).
Then we create a physical volume for LVM:

lvm> pvcreate /dev/sdb1
Physical volume "/dev/sdb1" successfully created

After that we can extend the volume group with this new physical volume:

lvm> vgextend VolGroup00 /dev/sdb1
Volume group "VolGroup00" successfully extended

Then extend the existing logical volume with 100% of the free space from the new physical volume:

lvm> lvextend -l +100%FREE /dev/VolGroup00/os
Extending logical volume os to 27.84 GB
Logical volume os successfully resized

That’s all, the existing logical volume is now extended with the new harddisk.
If you have a ext3 filesystem on this logical volume you can extend this online using the command:

[root@dbdev01 ~]# resize2fs /dev/VolGroup00/os
resize2fs 1.39 (29-May-2006)
Filesystem at /dev/VolGroup00/os is mounted on /; on-line resizing required
Performing an on-line resize of /dev/VolGroup00/os to 7299072 (4k) blocks.
The filesystem on /dev/VolGroup00/os is now 7299072 blocks long.