Yes, you can certainly modify existing reports. See page 166+ of the Administrator Guide (Creating and Modifying Reports) to bring the report into Report Writer.
I'm sure some SQL expert in the community will be able to help with the statements!
I think i phrased my question incorrectly as I know you can edit the reports, my question was rather to find out if you could and how add a count column which showed the number of times the device was down in a period on the existing report. However i believe that the structure of the report would not allow this as it is not an aggregate report and represents the actual time the devie went down.
I think a separate report would need to be created that shows the number of times a device was unresponsive in a time period.