Deep wrinkles will need to be part of the model.
The bump map technique you describe is the preferred method for dealing with shallower wrinkles.
Personally, I would set up a layered texture with eack set of wrinkles you'll need mapped to a different layer, with the blending mode set to Multiply or Darken. Then control the blending by using Set Driven Key to affect the Alpha values of the different layers.
So, for example, if you have a blendshape target where the character lifts his eyebrows in surprise... you would create a wrinkled-forehead bump map and make it part of your layered shader. Then set driven keys so that when that blendshape is at level 0, that layer's Alpha is also 0... and when the blendshape is at level 1, the Alpha is at 1 (or however you want to fine-tune it).
Good luck!
-Steve
