Hi
Well you have to know how to get your approver (agent) for each release code which is something like I_FRGCO in the exit.
Once you get your agent depending on your business logic you add them to the actor_tab;
I personnally like using determination rule (PFAC) to apply my logic
you can call a determination rule by using a code like this where 90200001 is the rule number
CALL FUNCTION 'RH_GET_ACTORS'
EXPORTING
act_object = 'AC90200001'
TABLES
actor_container = ac_container
actor_tab = actor_tab.
* ERROR_TAB =
* EXCEPTIONS
* NO_ACTIVE_PLVAR = 1
* NO_ACTOR_FOUND = 2
* EXCEPTION_OF_ROLE_RAISED = 3
* NO_VALID_AGENT_DETERMINED = 4
* NO_CONTAINER = 5
* OTHERS = 6
For MRP agent check the rule 20000054, check the description to know what you need to do for it to find the agent.
Please check the following
SAP Workflow Agent Determination
There are a lot of information available on the SCN about the release strategy and rule determination also if you really want to work on workflow my advice would be to go on
Practical Workflow for SAP. von Jocelyn Dart, Susan K - by SAP PRESS and read the book
Cheers
Stephane