Still no indication of where this 'field' is located, but with respect to your most recent message
Item.Body = Item.UserProperties("Company") & vbCrLf & Item.Body
will put the value before the body.
If you want to edit the body you need to use the Word message editor e.g.
Sub Example()
Dim olEmail As Outlook.MailItem
Dim olInsp As Outlook.Inspector
Dim wdDoc As Object
Dim oRng As Object

    On Error Resume Next
    Set olEmail = ActiveInspector.CurrentItem
    With olEmail
        Set olInsp = .GetInspector
        Set wdDoc = olInsp.WordEditor
        Set oRng = wdDoc.Range(0, 0)
        'oRng is the start of the message
        oRng.Text = olEmail.UserProperties("Company") & vbCr 'Your field result
        .Display
        .sEnd
    End With
lbl_Exit:
    Set olEmail = Nothing
    Set olInsp = Nothing
    Set wdDoc = Nothing
    Set oRng = Nothing
    Exit Sub
End Sub