From where do the encodings supported by Terminal come?
At the bottom of the encodings menu you should see "Customize List".  Select that and it will give you the full list of 100 or so available.  Is what you need not on that?

A reference for Apple's Text Encoding Converter, which probably provides this function, is at

https://developer.apple.com/library/mac/documentation/Carbon/reference/Text_Encodin_sion_Manager/Text_Encodin_sion_Manager.pdf

iconv includes ucs-2 in case that is of any use:

https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man3/iconv_open.3.html