How can I extend my pre-built cabinets to the ceiling?
Cabinets aren't usually extended to the ceiling. Either a soffit is built before cabinets are installed, or crown is installed directly on the cabinets (well below ceiling height). In your case, crown around the full-height cabinet and on the lower cabinets might look great, with all that detail and height variation to add interest.

However, if you're set on doing this, I'd build shells out of white melamine that are slightly smaller than the cabinet footprint, so as to avoid flush joints, and that reach to about 1/2" from the ceiling. This reduced height is to avoid the need to custom-fit to the ceiling. Then you can install your crown to the shells. 

 _________________________________________________ &lt;-- ceiling
    \\  _                                 _  //
     \\:_:_______________________________:_:// &lt;-- crown molding
       | :                               : | 
       | :                               : |
       | :                               : | &lt;-- shell (sides shown)
      _|_:_______________________________:_|_
     |                                       |
     |                                       | &lt;-- cabinet
     |                                       |


If you wanted to build the shells exactly to cabinet size you'd need to wrap the joint seam with some small trim to conceal the seam.