Actually, there is no easy way to convert char[] to string. You must create a bean a export this bean to the jbpm.cfg.xml, like <object name="myConverterBean" class="my.converterBean"/>, then use this bean to convert char[] to String. like #{myConverterBean.convert(charArray)}.
Hmm~, I see. Seems we should provide a way to auto-detect how to store long string.