This Outlook macro will allow you to append tags to the subject line of any message currently selected. To add a macro you must enable the developer menu in Outlook's options. This is based on code I found here http://www.vbaexpress.com/forum/showthread.

subject.bas

Attribute VB_Name = "Subject"
Sub AppendTags()
    On Error Resume Next
    Dim MsgColl As Object
    Dim msg As Outlook.mailItem
    Dim objNS As Outlook.NameSpace
    Dim i As Long
    Dim subjectname As String
     
    Select Case TypeName(Application.ActiveWindow)
    Case "Explorer"
         ' a collection of selected items
        Set MsgColl = ActiveExplorer.Selection
    Case "Inspector"
         ' only one item was selected
        Set msg = ActiveInspector.CurrentItem
    End Select
    On Error GoTo 0
     
    If (MsgColl Is Nothing) And (msg Is Nothing) Then
        GoTo ExitProc
    End If
    subjectname = InputBox("What would you like to append to the subject?")
    If Not MsgColl Is Nothing Then
        For i = 1 To MsgColl.Count
             ' set an obj reference to each mail item so we can move it
            Set msg = MsgColl.Item(i)
            With msg
                .Subject = .Subject & " " & subjectname
                .Save
            End With
        Next i
    ElseIf Not msg Is Nothing Then
        msg.Subject = msg.Subject & " " & subjectname
    End If
ExitProc:
    Set msg = Nothing
    Set MsgColl = Nothing
    Set olMyFldr = Nothing
    Set objNS = Nothing
End Sub