I just answered this question from another user over in the support queue. We don't add new modes until they have been added by the ADIF committee.
The modes they currently support are here:
http://www.adif.org/304/ADIF_304.htm#Mode_EnumerationTypically the author of a new mode, or the author of the program that supports the mode, contacts the ADIF group with a description of the new mode and requests that it be added, once that is done, we will add it here.
73, Dave - VA3KAB