I don't think abc supports unicode, otherwise you could use them directly into your lyrics, so you can't use those unicode symbols into the abc online converter. (and I can't display them into this forum either...)
The other way to go is to use abcm2ps and fontforge to create your extra symbols.
You can see an exemple there:http://anamnese.online.fr/site2/index.p ... =abc_perse
(choose "english version" on top of the page). It's quite complicated, but you can customise almost everything in your abc output. (sorry the file definition comments are in French)
You can get a more deep customisation there:http://anamnese.online.fr/site2/index.p ... enaissance