I have been using organic, all natural no-stick cooking spray (organic canola oil) for cooking recently. I know it is good for frying but I'd like to know if I can extend the usage of it. Can I use it as a salad dressing?
It won't have much flavor: unlike Olive, Canola Oil is rather bland, which makes it good for frying and lubricating when you don't want to add to the taste. You'll probably just end up with oily salad.
Well, it won't kill you.
It probably is going to just taste like oily salad. Additionally, spray oil is rather expensive in terms of oil - if you're looking for oil on salad at least go for olive or another flavorful oil in normal liquid form.
Because it is a neutral medium, canola is not ideal for flavoring a salad with the oil in and of itself. However, neutral oils can be used as carriers for other flavors.
The spray application (assuming you are referring to a pre-packaged non-stick spray) will effectively nullify your ability to infuse flavors. Also, if you are using something like Pam canola cooking spray, there will be corn alcohol that won't cook off as it would in a baking/frying application (unless you saute your lettuce, of course), which may contribute a non-trivial flavor or effect.
Perhaps you find spray oil convenient, and canola preferable to other more flavorful oils. In this case you can use canola oil to add flavor. If you are using a pump-spray type mister you could infuse the oil with other flavors (herbs, other oils, spices, or garlic).