You can create a NURBS surface from a bitmap relief with ZSurf (free).
But really, trying to do this organic, fine detailed type of surface is not a strong suit of NURBS. Your files will be very heavy.
If you want to start doing "Bark", you might want to explore a poly modeler that can save a good STL to use as the object to cut...
You may be able to look here for results:
Sculptris Alpha 6: It's HERE!!
Then you could use Meshlab to open the obj and save as an STL.