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If you are using FP extensions to process your forms, there should be no reason why you can't use FP includes to add fields to multiple forms. Not sure though if you can use regular SSI such as SHTML, asp or php includes. The biggest factor, I would imagine, is if you have a landing page with form results since FP has a results tag for each field. Bottom line - as long as A = B (so to speak) there should be no problem.
Marshall
I don't think you can use SHTML either from what I can remember.
Frontpage does have a Frontpage Page Include that you can use. The Frontpage Page Include is handled by Frontpage and not Frontpage Server Extensions which might be better for you.
Otherwise, you can use PHP / ASP to process the form. This is why I started using the JMail email component actually to email my forms so I could use ASP includes.
-Corey