You need to set up separate Service Levels for each set of support hours. Then you can associate these with their own SLAs and the SLAs with Services. For example you might have "Service1" and "Service1 (24x7)" and use the 24x7 one when logging out of hours.
I have tried setting up a service and an SLA with the 24x7 Service Level attatched to it. Then created a rule that when the priority is selected it changes the Service on the call. When it does this the Service Level does not change from the default and is greyed out on the call. Is there some kind of reason for this?