I was just wondering why I can't sculpt on my solid model is a way to turn a solid model into one that you can sculpt on? Can you combine or add a sculpted part onto a solid without worrying about having multiple vertices in the same space and if it is a problem how can you resolve this?
To answer the first part of your question, any mesh can be sculpted on in Blender. The way that sculpting works is by moving the positions of vertices, so your model must have vertices where you are sculpting otherwise nothing will happen.
But, if a Subdivision Surface or Multiresolution modifier is added, the model will gain sufficient geometry to be easily sculptable.
As for having multiple vertices in the same location, this can be resolved by pressing W > Remove Doubles in edit mode (in 2.79).