Did you know that you could set up dynamic schedules for your SLA? Apart from the static schedule source, you can create a custom reference field on your target table (eg: Incident) referring to the Schedules (cmn_schedule) table and then use it within your definition.

The custom schedule field should be populated via a business rule when your incident is generated depending on your requirement. When the SLA is created, the schedule applied to your incident record will be used to determine the SLA targets.