If there is a consistent pattern to what you want to replace - e,g, all text
that is to be replaced with a merge field begins with an uppercase X and the
letters in the text are all in uppercase and the corresponding mergefield
has the X replaced by CM, then it should be possible using code such as the
following:

Dim drange As Range, dfield As String
Selection.HomeKey wdStory
Selection.Find.ClearFormatting
With Selection.Find
   Do While .Execute(FindText:="X[A-X]{1,}", MatchWildcards:=True,
Wrap:=wdFindContinue, Forward:=True) = True
       Set drange = Selection.Range
       dfield = "CM" & Mid(drange, 2)
       ActiveDocument.Fields.Add Range:=drange, Type:=wdFieldMergeField,
Text:=dfield
   Loop
End With