How can I add major geometry to a mesh in Sculpt mode? Let me clarify the question.  I've had this happen numerous times, so I need to find a way to solve this 'problem'.  I'll start with the default scene, everything deleted.  Blank slate.  Create a 'round cube' primitive.  Give it a multiresolution modifier.  Go into Sculpt mode.  Start sculpting on it.  Eventually, I realize I have a good head-ish mesh for something.  But.. I've already made major changes to the mesh, in sculpt mode.  Now, I want to give the head a body.  But, every method I've tried just.. flat out fails.  Using the grab brush eventually pulls out really thin shapes that just turn into ugly reversed normals and jaggy edges that I can never fully fix without just undoing the entire operation.  Adding geometry by adding more primitive objects, causes them to get affected by the modifier so when in sculpt mode they look horrible, and they do not join to the original shape, leaving a gap between what should be a connected head and neck/body.  What am I missing?  

Using the Clay strips brush to build it up doesn't work, it runs out of resolution too quickly and Blender does not have a re-dynamesh option like Zbrush does, once you've begun sculpting.  Which is to say, I know there's the multiresolution modifier, but applying it farther just subdivides the ENTIRE mesh, which often produces unwanted results wherever you are not trying to work.  So really.. what can I do in this situation?  A lot of times the inspiration strikes to make something and I have to just get it into Sculpt mode as quickly as possible to get my idea out.. so I don't often have the luxury of going and blocking out an entire figure.

EDIT:  I guess it helps to add, I =cannot= use Dynamic Topology sculpting, on account of the models being intended for use with bone-based animations.  Triangle-covered surfaces don't tend to deform properly with bones!
Have you tried applying the multires modifier when you are happy with it? Then adding your primitives by joining (with all my desired mesh objects I wish to join selected I use Ctrl+J  to join them together)?

I enable dynotopo once I join my meshes to each other.

Depending on how rough/fine you want your sculpt to be at this point, will depend on how you set dynotopo's value. I personally like to use 5 or 6px.

I then lighten up the influence of my brush by setting it to near 0.0 with Shift+F and then dragging my mouse away from the center and click when I'm at my desired value. 

Then I lightly cover my transition points between my sculpt and my primitives with my sculpt brush (If this is slightly above 0.0 it will seem like it is not doing much, however you are adding mesh density).

Then I hold Shift and go back over and smooth the transitions.