Hi Iuri,
I have just recently "upgraded" an HTML mockup with a nice looking form to a ad_form by simply
a) copy-pasting the HTML mockup "as-is" into the ADP file
b) only replacing the <form> tag by <formtemplate> and the input elements by <formwidget>.
c) adding a corresponding TCL file with an ad_form that has a simple element stub defined for each input in the form. (i.e. more or less only my_input_name:text(text),optional)
Because of the fact, that I have only used ad_form with empty <formtemplate> tags in the past (where the whole form look and feel is generated automatically), I was surprised how easy it was to get the basic functionality working. In order to provide server-side validation, I later added a handful of <formerror> tags here and there. There may be useful features of ad_form which can't be used when going this way, but for simple cases, it could be a first step for a migration.
What have you tried? What was the showstopper?