Unfortunately iconv has no way of finding out what encodings are supported, so Geany has a hard coded list of fairly common ones that its hoped all iconvs support.
Geany can also open files with encodings that are not in its list but detectable by iconv.
Somebody might look at allowing encodings to be entered manually in the open dialog, but its unlikely to be any regular contributors since they are all Unicodeists. Pull requests are welcome.