Hi Anu,
If your additional data has only one value would map and need to query fast with index then you can suggest System Parameter e-g sales order, shipment, container id etc, otherwise you can check control parameter and info parameter.
Check this SAP help link to get more detail Parameters - SAP Event Management (SAP EM) - SAP Library
Discussion - system , control and info parameter | SCN
"2. I need to provide a utility functionality(via web service) to fetch the event data (for an order) and I need this additional data to be retrieved and send back along with actual event data of the order to the web service caller. Which functionality I shall use in EM to fetch actual event data? is there any BAPI or web service available already in standard?".
SAP EM has WEB UI service where you can give access to user, once you configured system, control and info parameters If you want to send back the data to external party via link or through email then you can use SAP EM Alert Framework which are readily available.
Regards
GGOPII