Mailmerge and legacy forms are mutually exclusive. I would suggest using content controls in the document. You can then even put merge fields in the content controls to allow the recipient to change incorrect data.
Insert Content Control Add-In includes an easy way to convert form fields to content controls.
If you then wish to mail merge and produce a form that has fields that may be edited, without disturbing any other content, then use
E-Mail Merge Add-in and run the following macro from the process to merge to documents or document attachments to e-mail (not PDF or to e-mail body).
Code:
Sub LockForm(odoc As Document)
Dim oCC As ContentControl
For Each oCC In odoc.ContentControls
oCC.Range.Editors.Add (wdEditorEveryone)
Next oCC
odoc.Protect Type:=wdAllowOnlyReading, NoReset:=True, Password:="password"
End Sub