Unfortunately, not according to the rules. It has been pointed out before what a poor example that is. The only rule mechanism for modelling how appropriate an ability is for an action is the improvisational modifier, which is always a penalty. Few would deem that Close Combat merits such a penalty when used to attack a dragon, so directly using a 'Dragonslayer Sword' ability (rather than using it as an augmentation) is unlikely to be advantageous (as the warrior is likely to have a larger Close Combat ability than Dragonslayer Sword ability).
I've been told that a playtest draft of the rules allowed you to assign a bonus to ability ratings if they were useful (or more useful) in only some situations. IIRC, David Dunham expressed regret that this rule was removed. It sounds as if this kind of rule would model a Dragonslayer Sword well. I hear a second edition of the rules is planned. Perhaps the authors could rexamine that discarded rule?
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