Add Time & Date in current cell without losing existing data

Trying to create a macro that will add the date & time & initials (i.e 8/26/09 2:34 PM JOD) into the current cell.

I've found plenty of macro's that will do this but it ends up deleting any existing text within the cell. I need to be able to add it in the middle of a text string.

Any help would be greatly appreciated

For example, the following macro will insert Date/Time into a field but it clears the contents of anything in the cell.

Sub Set_DateTime()
ActiveCell.Value = Format(Now(), "m/dd/yyyy h:mmAM/PM")
End Sub

This is an example of what my notes field would look like:
8/26/09 2:33 PM JOC Tested Ok. 8/27/09 3:33 PM ABC Sent tech to regional.

How can i automatically add 8/27/09 3:33 PM ABC without clearing the existing data in the cell?

Thanks

Sorry for not putting the macro within a code. Still no luck with this one?

Thank you shg! One issue I'm having is that when you enter this time stamp, it unbolds all previous stamps in the cell. Is it possible to not affect the pre-existing text format? Also, is it possible to push the old text down to the line below the new stamp, something similar to what I've pasted below? I have also included what my code currently looks like.

"05/19/2014 (SM): Allocated.
05/18/2014 (SM): Uploaded."

Private Sub CommandButton1_Click()
Dim MessageBox
MessageBox = MsgBox(Prompt:="Add Comment?", Buttons:=vbYesNo)
If MessageBox = vbYes Then
With ActiveCell
    .Font.Bold = False
    .Value = Format(Now(), "mm/dd/yyyy") & " (SM):" & .Text
    .Characters(1, 15).Font.Bold = True
End With
If MessageBox = vbNo Then Exit Sub
End If
End Sub

Thanks you so much!