One of my hiking buddies uses this pack, and there are definitely a lot of ways you could modify it and cut out weight. Some things would be easy, like cutting off unnecessary things. Adding things to it can get a little trickier, especially if the frame can't be removed. Adding your own extension collar would probably be easy. When you cut off the existing one, leave a little extra fabric to work with. If you cut too close to anything, it will be harder to sew on the new collar.
If you want to add a mesh stow pocket, it may be very difficult if you can't remove the frame. You'd probably have to sew it by hand, but if the frame comes out somehow, you may be able to use a sewing machine, but I'd imagine it still wouldn't be easy without much sewing experience.
I've found after modifying some of my gear and making some of my own, that making my own ends up being easier than modifying things that are already made. This is because it's easier to sew individual panels than it is to figure out how to get a bunch of fabric already sewn together between the feed dog and pressure foot. And you have to constantly figure out how to keep fabric from the pack out of the way as the fabric gets sewed.
If you don't have any sewing experience, I'd recommend starting out by making a (cheap) prototype pack and gaining sewing experience before modifying a pack you really like. You can seam rip mistakes, but it may end up being quite frustrating. I know from experience.
Either way, modifying and customizing your gear and making your own is thoroughly enjoyable once you get the hang of it, but first practice on things you don't care much about.