Outlook uses Outlook Express as its news reader so you can try this
(courtesy of Ottmar Freudenberger in the OE group):

To reply at the bottom you'll need to manipulate a registry key:

HKCU\Identities\{The OE Identity's GUID}\Software\Microsoft\Outlook
Express\5.0

create a new DWORD value named "Reply At End" and associate the
content "1" (one). That will lead to bottom postings with the
signature (if any) being placed at the very end.

HTH