Well it is possible 🙂 with a bit of hacking – but thats what its all about, isn’t it !
I actually used an existing working VM that had been built on VMware.  I will test building one from scratch on VirtualBox but not 100% sure it will work….
1) Get the existing VMDK and copy it to your Virtual Box machine repository
2) Create a new VM using a Linux, Redhat, 1024MB RAM etc machine and set the disk as the VMDK
3) Modify the machine – I removed the sound card
4) Locate the machines XML config file.   The directory is set in VirtualBox’es config.  Edit it and add the following:

Add the following lines to it in the "<ExtraData> </ExtraData>" section:
<ExtraDataItem name="VBoxInternal/Devices/pcbios/0/Config/DmiBIOSVersion" value="6 "/>
<ExtraDataItem name="VBoxInternal/Devices/pcbios/0/Config/DmiSystemVendor" value="VMware"/>
<ExtraDataItem name="VBoxInternal/Devices/pcbios/0/Config/DmiBIOSVendor" value="Phoenix Technologies LTD"/>
<ExtraDataItem name="VBoxInternal/Devices/pcbios/0/Config/DmiSystemProduct" value="VMware Virtual Platform"/> 

(That bits from http://ubuntuforums.org/showthread.php?t=1029144)
5) Next follow the guide here http://www.blindhog.net/how-to-get-root-access-on-call-manager-56-server/ to get root access.  Make sure you use the CentOS Disk 1 rather than the Live Disk…….grrrrr
6) Edit the /usr/local/bin/base_scripts/hardware_check.sh script to look like this:
We essentially comment out the hardware check and validation 🙂

function check_deployment()
    local tmp_deployment
    # Check the deployment
#    isHardwareValidForDeployment $tmp_deployment
#    rc=$?
#   if [ $rc -ne 0 ]; then
#         log info "$tmp_deployment deployment Not Supported"
#         return 1
#    fi
    # Deployment is supported by this hardware
    log info "$tmp_deployment deployment Is Supported"
    return 0

 7) Save it and reboot
8) The VM should come up and work !