Ok I just started chapter 4, and I refused the Pirate Lord's offer to get into Spellhold. Instead I killed Perth the Adept and got the ward stone.

The ward stone let me enter Spellhold, but in the foyer, there are 2 doors. Both doors are magically locked, and cannot be "picked" or "knocked".

Does anyone know how these doors can be opened?

Another thing... I asked Sime how to get in, and she suggested I talk to someone named Sanig (Or some such name). Does anyone know who/where Sanig is?

Someone should have appeared in the hallway and given you a tour of the place. Did you just find Perth the Adept and kill him or were you directed to him. Have you done Galvena's quest? Maybe you haven't triggered off the sequence by which the guy appears.