No, you can't use Sonar as a plugin inside of FL Studio but you can use FL Studio as rewire inside Sonar.
Some plugins included with Cakewalk can be used in FL Studio.
The plugins that DO work in FL Studio:
The SI series, Acoustic Strum Sessions, Addictive Drums, Melodyne Essentials, Dimension Pro, Rapture, DropZone, Cakewalk Sound Center, Session Drummer, Channel Tools, Boost11, Z3ta, and some other Cakewalk developed plugins will work.
The plugins that DON'T work in FL Studio:
Nomad Factory plugins, True Piano, Breverb, TH2, Lounge Lizard, and a few others won't work.
If you have specific questions about using plugin X or Y inside FL Studio, please continue technical questions on the forum.
Image-Line Support Team