This feature has been added to version 1.0.1 of the Orion Management Pack which I believe is now available for download on the solarwinds website.
Once you have installed the new version, run the configuration wizard to completion which will update the management packs with the new features.
Once the management packs have been updated:
open up the OpsManager Operations Console
Go to the Authoring section and select Rules.
In the Look For area type 'Create Response Time Alert' (without the quotes) and click Find Now
Right click on the Create Response Time Alert that is under the Orion Node type.
Select Overrides->Override the Rule->For All Objects of Type: Orion Node
Check the box next to Threshold Value and change the value under Override Setting (the value is measured in milliseconds)
Select the Management Pack you want to store the override in and click Apply.
Note: If you have the rules scoped so that it only shows the rules for certain types you will either need to clear the scope so that it shows all of the rules before performing the search or you can add the Orion Node type to the scope.