Assigning value to key field in table

   First off I don't have a choice about the structure of these tables because I am programming against a database used by other apps.  I have two tables that are related.

Event - Primary Key EventId

EventStep which is a child of Event and has the EventId + StepID as the Primary Key and a foreign key of EventId back to the parent.  

When I create the Event record, I want to retrieve the next value from a table which holds the next EventID.

When the step is created I want to do the following.

public static EventStep Create(PersistenceManager pManager, Int32 eventId, Int32 stepId)

{ 
EventStep aEventStep = pManager.CreateEntity<EventStep>();

aEventStep.EventID = eventId;

aEventStep.EventStepID = stepId;
aEventStep.TSCreated = DateTime.Today;

aEventStep.DateOccured = DateTime.Today;

pManager.AddToManager();

return aEventStep;

}

It tells me that I can't do this because EventId and EventStepId are read-only fields.  How do I get around this, other than changing the database which isn't an option at this point in time.
Bill