Hi Jason,
Currently that is not something we can do as Impact is part of the calculation for Priority (Impact + Urgency = Priority - based on the priority calc matrix). The SLA engine is currently designed around priorities. However, you could try something like using service contracts with service levels attached and have those trigger via an event/condition - that being, impact is set to a certain value. You would need to have it as an auto-event that is attached to every ticket that gets opened. Just a word of caution on that that though - it can create a lot of overhead work for the system in the background making for possible performance impacts, so just keep that in mind.
Thanks,
Jon I.