Is it possible for pressure sores to spread to unaffected areas?

Yes. Pressure sores occur in patients who either lack sensation (e.g. Spinal cord injury patients) or the ability to reposition themselves (elderly or comatose patients). In general, repositioning should be done every 2 hours to prevent the development of pressure sores.) with infection, tissue loss may spread to other surrounding areas.
Pressure Ulcers. Pressure ulcer development is a sign of prolonged pressure injury to an area of skin. Any area that has increased pressure for a prolonged time is susceptible to this injury. Often, contiguous areas are involved, like in the sacral and coccyx area. The injury does not 'spread'.
Yes . Infection can cause pressure sores to appear to be spreading. Also if the patient is turned to keep pressure off the effected area a new sore can be created if the patient is not turned properly.