I believe they have every timezone already, or at least the vast majority of them. Which have you noticed are missing? Some may be under different names that what you use, but if you look at the times, they're pretty much all there. You can change your timezone and view all the available ones here - Forum/ucp.php?i=165
The time zone setting is there to determine the time stamps displayed for you around the site (on forum posts, trades, etc.), so I don't see an "other" option working because then the site wouldn't know what time to show.
I think there's an option available for every time zone already (try checking for the "UTC +/-" number rather than a specific name or location), but if you do notice a time missing, please feel free to report it.