Let's a :APPT_WARNTIME: property override `appt-message-warning-time' in `org-agenda-to-appt'.
* org.el (org-speed-commands-default): New speedy command to
quickly add the :APPT_WARNTIME: property.
* org-agenda.el (org-agenda-to-appt): Use the :APPT_WARNTIME:
property to override `appt-message-warning-time' when adding
an appointment from an entry.
* org.texi (Weekly/daily agenda): Mention APPT_WARNTIME and
its use in `org-agenda-to-appt'.
This feature has been suggested, along with preliminary patches,
by Ivan Kanis. Thanks!