In the Textmate grammar you can inject rules into an existing scope (for instance if you want regular expressions to have their own grammar in python code) by using the grammar rule
'injections': 'scope.selector.statements': 'patterns':[ ... ]
If you want the new injected rules to take president over the rules in the grammar you are injecting into you can modify the scope selector with
L: so the rules are injected "to
'injections': 'L:scope.selector.statements': 'patterns':[ ... ]
the left" of the, i.e., in front of the existing rules. To the extent I’ve been able to test, this last point does not work in Atom. No matter what I try, I am unable to get injected rules to have higher priority than the rules in the grammar being injected into.