Given the fact that the jenkins plugin bundles findbugs and uses it to determine the rank of a bug instance it is not possible to override the default values. (not that this is easy to do with findbugs to begin with).

It would be nice to be able to change the rank of a bug pattern in the analysis of the jenkins plugin. This way you can gradually increase the software quality by making certain types of bugs more critical to the build result (without needing to fix 100s of bug instances in one go).