Set precise default portal timezone

From /@@dateandtime-controlpanel the default portal timezone can be set, but the list is limited.
I'd like to set the time to my very own city, not the next largest city (in the same timezone).
Is there a way to set this timezone, maybe from the ZMI or a XML file in my policy package?

The lists of timezones in the control panel are from two vocabularies:
(see class IDateAndTimeSchema in Products.CMFPlone.interfaces.controlpanel)

The vocabularies are defined in plone/app/vocabularies/configure.zcml

I would like to set to
without modifying the source code of How?

What is the usecase?

I want events to have a timezone with my city as the location; I want "America/Montreal", not "America/Toronto". The default portal timezone should be allowed to use any available variation of timezones.

I got help once with something similar, maybe it can be useful:

The implementation at

is the base for the available timezones.

You can create a custom vocabulary as you like - as long the name of the timezone is a valid one from pytz.all_timezones list

But why it this important having an alias for existing timezone offering the same TZ setting?

So that the timezone of events shows as happening in a specific city; that's pure chauvinism at work, and that could be why timezone aliases are for, unless I'm mistaken.