Assuming column A has the text values.

In B1 enter  ="\" & A1

Double-click on fill handle of B1 to copy down.

Copy then paste special(in place)>values>OK>Esc.

Delete original column A if desired.

Or you could use a macro to do in place with no formulas.

Sub Add_Text_Left()
Dim cell As Range
Dim moretext As String
Dim thisrng As Range
   On Error GoTo endit
   Set thisrng = Range(ActiveCell.Address & "," & Selection.Address) _
           .SpecialCells(xlCellTypeConstants, xlTextValues)
   moretext = InputBox("Enter your Text")
   For Each cell In thisrng
       cell.Value = moretext & cell.Value
   Next
   Exit Sub
endit:
   MsgBox "only formulas in range"
End Sub

Gord Dibben  MS Excel MVP