You could use the HYPERLINK function in adjacent cells ?
B1: =HYPERLINK("mailto:"&A1&"?subject=Subject&Body=Hello",A1)
If you do want VBA then assuming existing values are constants (curious as to why they're not already hyperlinks by default) then
Public Sub Example()
Dim rngCell As Range
For Each rngCell In Selection.SpecialCells(xlCellTypeConstants)
With rngCell
If .Hyperlinks.Count Then .Hyperlinks(1).Delete
.Hyperlinks.Add rngCell, "mailto:" & rngCell.Value, , , rngCell.Value
End With
Next rngCell
End Sub
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