"Extending" syntax files / overriding keyword with match

Hi, I'm extending the css.vim syntax file for use with a CSS framework
using "runtime! syntax/css.vim". I've got it mostly working, but I
need override some defined group names that are defined as keywords,
and change them to use match. For example, if the following was in
the base syntax file:

syn keyword fooGroup foo bar baz

and I want to override it as:

syn match fooGroup /somepattern/

Is there a way to "unset" a group name as if it were never defined?
Or is there at least a way to give match precedence over the keyword
for that group name?

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