If for example...
- Your data validated cell that you want to append with "New" is cell A1 on Sheet1
- Your dynamic list is named "MyList", housed on Sheet2
...then see if this helps. Right click on your Sheet1 sheet tab, left click on View Code, and paste the following procedure into the large white area that is the worksheet module. Press Alt+Q to return to the worksheet. Modify for the actual sheet names, target address, and named range.
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Cells.Count > 1 Then Exit Sub
If Target.Address <> "$A$1" Then Exit Sub
Application.ScreenUpdating = False
Dim cell As Range, strList$, strEnd$
strList = ""
For Each cell In Sheets("Sheet2").Range("MyList")
strList = strList & cell.Value & ","
strEnd = "New," & Mid(strList, 1, Len(strList) - 1)
.Add Type:=xlValidateList, _
.IgnoreBlank = True
.InCellDropdown = True
.ErrorTitle = "No such animal !!"
.ErrorMessage = "Please select a valid item" & Chr(10) & _
"from the drop-down list."
.ShowError = True
Application.ScreenUpdating = True