Is there a way to balance a wet and dry mix of an FX plugin in SONAR and what are the ways to do it?
For instance, there's a plugin called Driver from Native Instruments and I would like to control how much the plugin affects the signal like a wet/dry knob does.

If the plugin lacks a wet/dry mix control, there are at least two ways to accomplish it. The easiest is to use an aux send to a bus and place the effect there. That's been the traditional method since the 60's, and requires no additional plugins.  Another way is to use a plugin that is also a VST host, such as Blue Cat Audio's PatchWork plugin.
 
 

Create a bus with the plugin in the fx rack, then use a send from the track(s) you want to use the effect. Send the bus output wherever you want it!

bitflipper
If the plugin lacks a wet/dry mix control, there are at least two ways to accomplish it. The easiest is to use an aux send to a bus and place the effect there. 

building on BF's idea, you can just use an Aux TRACK with the effect on it.
This has the advantage that you can put the Aux Track right next to the original "dry" track in the Tracks and Console views.  Makes mixing easier.