override system mime type

hi members,
how do you override a system mime type?
in order to have a custom icon in my file browser (thunar), I would simply
like to add <icon name="media-floppy"/> to the application/x-vmdk-disk mime
type defined in /usr/share/mime/ packages/freedesktop.org.xml : should I
use/write the full type definition or is there a possible shortcut ?
I just searched the archives but couldn't find anything about it.
regards, lacsaP.