How can I add more columns into my Watchlist ?

First, open the watch list and press the right mouse button on any column and then open "Column Chooser". Now, you can easily add the desired column by dragging it into the Watchlist.

In case you want to add additional columns in more than one Watchlist, you could set the Watchlist to be a default one. Just press the right mouse button on the title bar of the Watchlist and press "Set as Default for All" which will make all existing Watchlists and all new ones the same as the default one. If you press "set as Default", only the newly created Watchlists will be exactly like the default one.