Hi Radhika,
the problem is... it's not that simple. The event-based approach of Process Observer (POB) provides a lot of advantages, including real-time capability and a generally more intuitive approach than the extraction-based procedures, but the downside is that accessing historical data, i.e. any data that was created before you switch on Process Observer, is difficult.
Generally bringing in old data must be considered a (small) migration project. The overall procedure consists of two parts: Based on a given process definition, you first need to extract the data and then put it into the log. Let's look at the second part first, so you can understand what kind of data you need (and because it is actually the simpler part): Process Observer allows to send events retroactively, so you can send events "from the past" (the history that you want to put into POB). Technically you can - and should - use the same interface as for the direct event instrumentation, see e.g. the blog entry Direct Event API for Logging Processes from SAP and Non-SAP Systems and detailed information in the workshop document.
So what is left is that you need to translate your historical data into events... which is the difficult part, but which is something you need to do on a case-by-case basis, e.g. your process starts with the creation of a sales order. You can read the sales order and the sales order (like most objects) provides the information when it was created. You can now translate this to task 114-21 (Sales Order - Create) and put that into the interface (adding the ID of the sales order and any other required information).
Obviously, extracting these events retroactively is not an easy task (which is why the POB approach is event-based rather than extraction based), and you will not be able to extract all data. But you may decide that for historical data you need less events, which may need to be reflected in you process definition.
Best regards,
Christoph