I use a digital piano as my controller, which doesn't have a mod or pitch bend wheel. On the rare instances where I'm auditioning softsynth patches and would like to hear what the mod wheel does, I usually find the controller assignments within the synth and adjust the corresponding parameter, which can sometimes be a pain. Rather than unplug the digital piano and plug in a synth as a controller, I was wondering if there was some sort of global on-screen mod wheel, or "Assignable CC Fader" I could control with the mouse as I was trying out the sounds with my other hand? Thanks in advance for any advice!

I don't believe there is.
An alternative (which is what I do) is to create pitch wheel midi clips a few measures long and store them in the browser. Then you can drag and drop those clips on to a Synth track allowing you to make real-time decisions.

That's a cool idea, Chuck!  A couple of questions, if I may:  If I wanted to hear what the mod wheel was doing as I was physically playing the synth (rather than after I recorded a few bars), couldn't I play the silent track consisting of the controller data and physically play the synth AS the silent controller track was also playing?  Also, I've never drawn CC data in a MIDI track, or saved a MIDI clip in the browser.  I assume that's all pretty straightforward stuff?  Not at the DAW right now...  Thanks for the idea!

You say you'd rather not plug in a synth as a controller, have you considered connecting both?
How are you connecting your piano to your interface?
If it's midi, a simple Midi Merge box would suffice.

Digital piano has a USB out, so that goes right into the computer. I have a MIDI-USB adapter that I could use for the synth (assuming I have a free USB port). Don't know the first thing about having Sonar recognize 2 simultaneous controllers; would there be a MIDI controllers preferences menu or something along those lines? Apologies for the questions, as I'm not at the DAW computer at the moment.

Is you sustain pedal continuous (or semi-continuous) ?
I do not think it can work well as as pitch bend, but I use it as a ModWheel. You will need MFX which converts CC 64 to CC 1, f.e. "CC Map" from http://www.tencrazy.com/gadgets/mfx/
 
Since my DP is not near my computer, I use my own MFX with Sustentto (middle) pedal switching Sustain between normal behavior and  ModWheel (in addition to 3 upper keys switching partches in synth...). But for basic operation "CC Map" should do the trick.

I have the same issue...I solve it by a variation on Bristol_Jonesey's Idea.  I have a small mini synth (microKorg) connected in via a midi to USB adaptor.   Then select "All Inputs Omni" on the input of all your midi tracks. 
 
https://www.iconnectivity.com/products/midi/mio/
 
 

Another solution to consider is the Beheringer x-touch mini.  Program a couple of the knobs to be Mod wheel and pitch bend
https://www.sweetwater.com/store/detail/XTouchMini?campaigntype=paidsearch&campaign=Computer%2520Audio-Control%2520Surfaces&adgroup=Behringer-Mixing%2520Control%2520Surfaces&keyword=behringer%2520x%2520touch%2520mini&placement=google&adpos=1t1&creative=175887564453&device=c&matchtype=b&network=g&gclid=CjwKEAjw9afOBRDW0Y2qsf_D3WkSJACVuJ78in8O52v48X7nG86cueCB3ytC7bq2Lyh7KgHEEuEukBoC9oHw_wcB
 
 

Thanks for all the suggestions, everyone! I'm going to try hooking up my Roland XP-30 to the computer using a MIDI-USB adaptor, then use the "All Inputs Omni" selection. I'll post back later on once I've had a chance to try this out. Thanks again.

All good here. Was able to hook the XP30 up to the computer and enable it in a Sonar, which now gives me a mod wheel. For some reason when both the digital piano and the synth are checked in Preferences-->MIDI-->Devices then NEITHER controller will trigger a MIDI track. But as long as I check either one or the other, whichever one I checked works fine. So I'll use the piano for most of my controller needs, but when I need the mod or pitch wheel it's a simple matter of a couple mouse clicks in the Preferences menu. And I'm sure there's a way to get them both working simultaneously, but no biggie.