Setting dock item icons so they aren't influenced by systemwide settings

Hi,

I downloaded numix-circle icons which are beautiful and look good in plank. The rest of the icons that come with it, though, aren't great. Is there a way to force the dock items to have numix-circles so I can change my system wide icon theme without disturbing plank? Perhaps an 'icon=/path/to/icon' line in the .dockitem file?

Thanks in advance,
James