Can validation error messages be modified using DW without affecting FB?

I have a form where I ask for a phone number and an extension. I also supply a place for the user to add a second phone number and extension. I validate on the first phone number because it's required. However, the primary phone's extension isn't required, nor is the second phone number and its extension.

Can I just use DW to edit the error messages in case they put in the wrong format and control the validation that way, and if so, will it interfere with FB in case I need to edit the form again? I'd hate to lose those messages for each of the states required, invalid format.

In other words I need the form to be flexible enough to show errors if they enter something invalid, but the second phone number and phone extensions are not required fields so FB will not do any server validations on those fields.

I wish there was a way to do server validations on everything that's entered in the form, but it appears I can't do that unless I click the validation as "required." Of course not everyone will have a second phone or an extension, so I can't make those fields required.

steve