The thing is, there's a difference between an expression pedal and CV. Control voltage is applied to a circuit to change a particular parameter. Very hard to implement. An expression pedal is merely an external potentiometer which passively changes a parameter.
Technically no, there is no difference. Control Voltage is merely a term used to communicate a standard method of communication between pedals, and one that is not as well known in the pedal world. Synths yes, pedals no. This doesn't mean that you won't fry a pedal by trying to supply it's EXP IN jack with control voltage - if you don't know, just don't do it.
If a pedal has control over a specific parameter via a pot that you turn, you are in fact changing the voltage used to control that parameter. Depending on the pedal you are using, the voltage required to change or control the parameter may vary. A Vox wah may require 0.5 V to control it's entire range, while a Morley wah may require 1.5 V - hypothetically, I don't know the actuals. Rocking your foot opens and closes the pot (sends Voltage to ground, or its destination) for the purpose of opening and closing the filter (changing the frequency).
For specifics about voltage look here:
http://en.wikipedia.org/wiki/Voltage
For specifics about potentiometers (pots / variable resistors) look here:
http://en.wikipedia.org/wiki/Potentiometer
Can we get into why it would be so hard to implement? What we are talking about is a standard run of the mill Vox wah-wah pedal. What are the specifics to why I couldn't disconnect the potentiometer from the circuit, and in its place have a jack where by an LFO controls what the pot would normally control?
The EHX HOG has an expression in jack that is used to plug an expression pedal in, the function of the pedal is to change the parameter you have it set to change. This in fact is not unlike a moogerfooger, where you simply plug an expression pedal into a specific jack to change a specific parameter. You essentially create a remote knob to control the parameter. A pot is a variable resistor, sending a voltage through it will change the voltage, unless fully open. This happens regardless of the circuit being active (added power) or passive (self generating power like a magnetic guitar pick-up).
The jacks on your moogerfoogers (EXP IN if you will) are designed to default to the permanently mounted knobs in the moogerfooger to define the parameters voltage when not in use. When you plug in the expression pedal, it now becomes the default, or master control knob if you will. The pedal's internal voltage is supplied to the jack / pot and back again. When you plug a "control voltage" into the EXP IN jacks on the back of your moogerfooger you are doing the exact same thing, supplying the parameter with a specific control voltage, that when changed, changes the effect of that specific parameter.
Some effects are designed so that excess voltage is sent to ground, meaning you don't fry your pedal when you supply it with a control voltage in the EXP IN jack. Some pedals are not set up this way, and send too much voltage to the internal circuitry, thus overheating, or damaging components.
So, if you have a parameter already available to change, you can theoretically implement a jack to allow an external control source. Due to the design of some pedals, both physically and electronically, adding a jack can be quite difficult. However, in the case of a wah pedal, you have access to everything you need to make it happen.
I bring up the HOG, because it works with MOOG control voltage defaults at maximum and minimum. You can try it with any pedal - at your own risk - using a CP-251. Route an LFO to the attenuator - set to middle, or 0 - route the output of the attenuator to the EXP IN jack on your pedal. Slowly turn the attenuators knob and see if it affects the effect. To find the maximum, send a constant voltage through the attenuator in the same manner. As you turn the attenuator control, you will hear the effect parameter stop changing. This is the maximum control voltage required for that particular pedal. DO NOT EXCEED IT.
Do some reading about how pots work, buy a volt meter, and test your wah out. Then go to the DIY forums for assistance, but try not to go empty handed, you get more help when you have done some leg work, and can discuss the solution further than "I want to use CV to control my Vox wah, tell me how".