I was able to find the location of the config file thanks to this information on maemo.org. The location of the config file is /var/lib/gconf/system/osso/connectivity/IAP/<Connection Name>. Although there is an XML key for "EAP_wpa_preshared_passphrase", changing the value doesn't seem to effect anything. Maemo.org says that changing this file will lead to "Inconsistent states" which seems to be true