I don't think that Sonar really needs a sampler. There are so many good of them on the market.
Most people working much with samples, own NI Kontakt, Motu MachFive, UVI Falcon or Air Structure.
And further don't forget that you can record samples in Sonar and alter them with many edit tools and effects. You can put your audio samples easily to the matrix view, Dropzone or Cyclone to play your samples. Another interesting one is to put your audio samples to Cakewalk's Rapture or Rapture Pro.
If you want more that Sonar provides with its tools and instruments, than invest to one of the good 3rd party samplers.
-Markus