Hello
Yes, you will have to add the approvers in the configuration against a Release group and Release code. In the workflow,the person responsible for Release of the PO is determined using rule 20000027 (FM - ME_REL_GET_RESPONSIBLE).
Make sure you COPY task TS20000166. If you look at this Task in PFTC, there are few terminating events defined in tab "Terminatiing Events". make sure that the custom task has these events too. Also check "Default rule" setting.
BUS2012 RELEASED
BUS2012 RESET
BUS2012 SIGNIFICANTLYCHANGED
Regards
Sandy