How to update system parameters passed as an event handler extension?

Hi,

There are two events in my scenario. one is close and second one is dispatch.

In Close event, I am passing few system parameters as event handler extension through xml in namevalue pair.Those are updated perfectly.

Secondly in dispatch event for the same eh guid, it will update the status in table /saptrx/eh_stat and it is doing it fine through activity ---> status_set.

Currently I have another requirement to update few parameter in the second event (dispatch), if i have missed any in first event.

For this , I am trying it to achieve it through PARAMETER_CHANGE activity but it is not updating the fields as all my parameters are system parameters.

1.Can system parameters be updated through PARAMETER CHANGE activity??

2.ACT_HDR_EXTENTION activity is updating the system parameters , if and only if passed through parameter ....merely hard-coded but not through xml...i.e namevaue pair...can this activity be used to update few parameters from xml .????

3.In PARAMETER CHANGE activity, the input parameter is i_eai...can anyone kindly suggest...in this parameter..where the extension values are passed?

4.Is there any staging or buffer table, in which xml datas are saved when any rule is triggered in OER/EM?

Thanks in advance