I have never posted a reply before but I happend to be playing with this same situation where I wanted to keep the existing path and add another one to it so this is what I came up with. It is very much like the one you wrote but this keeps the existing path and will add another path to the toolpalette paths
Code:
Sub ToolPalettePath()
'This example changes the tool palette path
Dim objPref As AcadPreferences
Dim strSupport As String
Dim strPath As String
Set objPref = Application.preferences
strSupport = objPref.Files.ToolPalettePath
strSupport = "<COPY AND PASTE THE EXISTING PATH HERE>" _
& ";" & "<PUT THE PATH YOU WANT TO ADD HERE>"
' You can add more paths using a underscore to continue to the next line
objPref.Files.ToolPalettePath = strSupport
End Sub