You might be in for a tough ride. In origin, Symbol used a non-standard character set that was essentially made up for the font, allotting characters to ASCII codepoints by visual similarity to Roman alphabet characters; there is a description of how that encoding maps to Unicode at http://unicode.org/Public/MAPPINGS/VENDORS/ADOBE/symbol.txt which you might find handy.
I imagine it might be tricky to persuade iconv to do what you want - you may have to resort to some scripting work to directly transcode each character.