I am not that familiar with WordPerfect or the layout of your forms and templates and
I don't know exactly what NESTFORM does, but my guess is that the closest thing(s) in Word would be:
a. use an INCLUDETEXT field to include (rich) text from another Word document or another file type for whcih Word has a (reliable) import converter. If necessary, use a bookmark name to retrieve part of document if, e.g. the text is only part of a Word document
b. use an INCLUDEPICTURE field if your letterhead is actually in graphical format
c. Keep all the necessary letterhead(s) in Autotexts and use Autotext or Autotext fields to insert them.All these approaches have their different advantages and disadvantages. In particular, if you need to save and archive Word documents "as produced", e.g. for legal reasons, you should be sure to convert any fields such as INCLUDETEXT/INCLUDEPICTURE to their results (e.g. ctrl-shift-F9) before archiving them.