Imagine your frame with no shock in it at all. The maximum travel you can get out of it is the maximum distance which the rear dropout moves (approx). You can only change the shock for another one with the same eye-eye length (so it fits in your frame), and therefore, whilst the travel may change a bit (if the new shock can compress a bit more), I don't think you'll be able to simply 'add a couple of inches' to the rear end of your bike.