Think you could play with the sub below to write the cell's contents into its
comment. Install* the sub, then just select the range on the sheet, & run the
sub.

*To install: Press Alt+F11 to go to VBE, click Insert>Module, then copy n
paste the sub into the code window (the empty white space on the right).
Press Alt+Q to get back to Excel.

Sub CellToComment()
   For Each cell In Selection
      If Len(cell) > 0 Then
         If cell.Comment Is Nothing Then
            cell.AddComment
         End If
         cell.Comment.Text Text:=cell.Value
      End If
   Next cell
End Sub