Yes, users can edit the deployed email signature when composing an email in Outlook. Currently it is not technically possible to lock the email signature in Outlook in the composing window.
If a user edits the signature when composing an email the changes will only apply to that particular email. The signature will revert to the deployed version on the next email the user composes.
Emails are saved in Sent Items including the signature, and any changes the user may have made to the signature.
Users cannot save changes to the deployed email signature in the Outlook Options dialog.