Nice little article available here on how to set a customized sort order in hierarchy prompts based off a TM1 datasource in report studio:
https://www-304.ibm.com/support/docview.wss?mynp=OCSS9RTN&mync=R&uid=swg21457443&myns=swgimgmt
Blog of IBM Cognos TM1, Cognos BI, Cognos Express, CDM and other related stuff.