Mail merge and form fields are mutually exclusive, however if your data is in Excel (or can be put into Excel format) you could use
http://www.gmayor.com/ManyToOne.htm.
In One to One mode, the add-in will allow you to retain the form fields (though it will not fill them) alongside merged data.
As the aim of the game appears to be to get users to validate the data you hold for them, you could create a table that contains the mail merged data you hold with form fields for any new data alongside and merge to e-mail.
In fact with a table you don't even need the form fields as it is easy enough to read the data back from a table to Excel - you'll find lots of help with that too on my web site.