Based on your limited description I would say that such a thing is likely possilbe. By "form" I assume you mean a protected (or online form). You could create and save the different deriavatives of the form as autotext entries. You could then present a UserForm containing the combobox when a new document is created using the template. When the user makes a selection and executes, you would used VBA code to unlock the form, insert the appropriate AutoText content, re-lock the form and present it to the user.
2. The UserForm is not in "IN" the document, but a separate dialog presented with and AutoNew macro in the template. See: http://gregmaxey.mvps.org/Create_and_employ_a_UserForm.htm