1)Can I add more columns in existing data feed & rerun the data feed jobs? what are the pros & cons of this approach?
I believe so, while I didn't save, the "Add Columns" function is available to me on an existing feed, whereas other things like changing the interval and delay, etc are not.
The pros/cons of this... well the pro of this is getting additional data that you may be missing... If you are trying to process raw data, this is something I am sure you will want/need to stay up to date with your tracking.
Con.. the only thing I can see, would be that whatever process is being used to read the feeds may not be set up to accept new fields.. now, if designed correctly, it shouldn't break anything, but work may have to be done to read and process the new fields, so you will have to communicate with your team about what changes are coming.
2)Can I create new data feed with new or all the required columns & run the jobs? will this capture the historical data?
You can create new jobs and run them, but if you are running an hourly continuous feed as per your screenshot, then no, it will not capture historical data. You would have to create a one-time "backfill" feed to accommodate historical data where you will have to choose a start and end time for what data you want to collect. You would then need to work with your team to figure out how to process and merge the historical data in.
3) How can I calculate Visits, Pageviews & unique Visitors if we don't have columns like post_visid_high, post_visid_low, visit_start_time_gmt, hit_source in the existing datafeed. We do have Visit_num & Visit_page_num. Any pointers?
Page Views isn't reliant on User Identification.. a page view is a page view, regardless. However, Visits and Unique Visitor calculations require identifying the user (i.e. post_visid_high, post_visid_low) and to properly identify the visit you will need the other fields you mentioned, as well as the visitor info...
Visit_page_num isn't enough to identify a visit, without being able to distinguish each user from each other.... especially if you have any oddities in your tracking that might need to be stitched.
