Display or actually become a negative number?

Display...........Format>Cells>Custom  -#,##0.00  but they would still be
positive numbers.

To actually become negative would require VBA.

You could use event code to change them to negative as you enter.

Private Sub Worksheet_Change(ByVal Target As Range)
 On Error GoTo ws_exit:
   Application.EnableEvents = False
   If Not Intersect(Target, Me.Range("A1:A10")) Is Nothing Then
       With Target
           If .Value <> "" Then
               .Value = .Value * -1
           End If
       End With
   End If
ws_exit:
   Application.EnableEvents = True
End Sub

Right-click on the sheet tab and "View Code"

Copy/paste the above into that module.

Edit the range of A1:A10 to suit.

Gord Dibben  MS Excel MVP