One of the great things about the English language is that it is flexible. There is no central body to determine what is a word and what isn't. If enough people use CAD as a verb, then it becomes a verb. No one body, for instance, made "googling" a verb, or made "spam" a type of e-mail. But they are now just as much a part of English as "searching" and "garbage". It is really a rather democratic process.
So if you want to use CAD as a verb, go ahead, and stand proudly as an advocate for CADding. There isn't a single word for "engaging in the process of design using computer assistance", and perhaps there is need for one. On the other hand, be prepared for some criticism from those who either don't feel the need for one... or who think that CADding is an unnecessary redundancy due to the fact that designing is a perfectly functioning existant verb that describes the core process and that the presence or absence of computers is largely irrelevant to the process.
Jason
P.S. There are several uses for the word "draft" and it's English counterpart "draught":
http://dictionary.reference.com/search?q=draft&db=luna