The definitions downloaded from LANDesk cannot be modified to ensure the integrity of the content.
What you can do instead is to clone the definition by right-clicking it and select "Clone...". That will clone the selected definition into a new custom definition that you can modify all you want.
Remember that the original content is of the "vulnerability" type and the cloned content is a "custom definition", so depending on your scan and repair settings, you might need to modify those to make sure they are scanned and fixed as well.
Thanks, I will try that.