VM setup custom xml code how to?

Hi All

I think there was a new feature to add XML changes so GUI changes didn't change them when editing here?

I am running the latest release candidate but I cant write in the custom field?

I would like to somehow add the following settings: (Not sure on how to go about this)
 

icon path:

  <metadata>
    <vmtemplate xmlns="unraid" name="CentOS" icon="/mnt/cache_appdata/domains/XPEnology_DSM_X/synology.png" os="centos" webui="" storage="default"/>
  </metadata>

And the controller to "1"

    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='writeback' discard='ignore'/>
      <source file='/mnt/cache_appdata/domains/XPEnology_DSM_X/vdisk2.qcow2'/>
      <target dev='hdd' bus='sata'/>
      <serial>vdisk2</serial>
      <address type='drive' controller='1' bus='0' target='0' unit='3'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='writeback' discard='ignore'/>
      <source file='/mnt/user/Photos_backup/vdisk_photo.qcow2'/>
      <target dev='hde' bus='sata'/>
      <serial>vdisk3</serial>
      <address type='drive' controller='1' bus='0' target='0' unit='4'/>
    </disk>

Is it possible to add them as above?
Or do I need some special format only for the part I dont want the UI to change?

Thanks

/Casperse