Can Cakewalk Control Hardware Synth Parameters?

You're right. Sonar can send out and receive MIDI data.

If you device doesn't speak MIDI you'll need some hardware to translate for you. You'll probably find it cheaper to get a controller keyboard and have all the sounds played in side Sonar via VSTi plugins and use the controller keyboard to write your midi messages recorded in Sonar. Controller keyboards are relatively cheap. I'm not sure how expensive MIDI to CV vice versa hardware would be not to mention availability.

Much easier and more straight forward get a $150 to $300 controller keyboard and never look back.

I recently sold my Pro One.
 
There are two CV ins on the back. One determines the note value, the other controls the filter only.
A midi-to-cv converter should at least control the note value (CV) and timing (gate) for one note. Beyond that, it may offer multiple CV outs and may be configurable to respond to MIDI mod wheel, or something else, to control a 2nd CV. So it's really up to the converter to provide the control you need. But the Pro One does not have a way to have its other parameters be controlled by CV or any other automated means. (A robotic knob-twister?)
 
I noticed a midi retrofit for the Pro One from Kentron. But I don't think it offers control of any other parameters.

Doepfer make MIDI to CV converter boxes that aren't very expensive, and the Dark Time sequencer is MIDI clockable and sends CV. As does the Boss/Roland Sq-1.

As for controlling parameters via external CV sources, if the synth doesn't have the necessary inputs it can't be done. A lack of MIDI control does have some advantages though. MIDI only has 128 values, assuming it's the usual 7bit variety. This means that MIDI "continuous controllers" actually aren't continuous at all but go in steps. This can be particularly noticable when manipulating filter frequency or pitch slides and you get a 'zipper' noise if the change is fast or a series of steps if the parameter is changing slowly. It's particularly noticable at high frequencies where the Hz difference between notes is greatest. CV control does away with that, though it does mean turning knobs.

The Pro One's a nice synth, my advice is to use it for its strengths and the quality of the sound. If you really want something very similar but entirely MIDI controllable the DSI Mopho and Prophets are the things to look at. Or maybe a more modern MIDI controlled synth to go with it.