You can apply this kind of validation to the form after you've created the form. Go to Insert->WebAssist->CSS Form Builder->Client/Server Validation and you can apply alphanumeric validation to the field.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.