There's no way to embed an actual field in a string, because a string is
just a sequence of characters. However, once the string is inserted in the
document, a part of the string can be converted into a field code, and the
resulting field replaces that part of the text. Here's a demo macro to show
what I mean:

Sub StringAndFieldDemo()
   ' choose any characters that won't
   ' occur in an input string
   Const Ldelim = "{"
   Const Rdelim = "}"

   Dim oDoc As Document
   Dim oTbl As Table
   Dim oRg As Range
   Dim oStr As String

   ' the delimiters in this string are
   ' ordinary characters surrounding
   ' what will become a field code
   oStr = "This => " & Ldelim & "DocProperty Author" _
           & Rdelim & " <= is a field."

   ' choose where to put it
   Set oDoc = Documents.Add
   Set oTbl = oDoc.Tables.Add(Range:=oDoc.Range, _
       numrows:=3, numcolumns:=3)
   Set oRg = oTbl.Cell(2, 2).Range

   ' insert the string
   oRg.Text = oStr

   ' contract the range to cover only
   ' the field code, and remove the delimiters
   With oRg
       .MoveStartUntil cset:=Ldelim, Count:=wdForward
       .MoveEndUntil cset:=Rdelim, Count:=wdBackward
       .Text = Replace(.Text, Ldelim, "")
       .Text = Replace(.Text, Rdelim, "")
   End With

   ' the magic statement:
   ' replace the text of the range with a field
   ' whose code is the text of the range
   oDoc.Fields.Add Range:=oRg, _
       Type:=wdFieldEmpty, Text:=oRg.Text

   oDoc.Fields.Update
End Sub