On my last outing I needed a bit more capacity than my Tarahumara had. My solution was to strap a MOLLE II sustainment pouch to the Tara's compression straps.
This worked pretty well and gave me an additional 432 cubic inches. However, if I was engaged in a strenuous activity where the pack might flop around it probably wouldn't be secure enough. Over on BCUSA, "Creaky Bones" posted how he attached a MOLLE II waist pack to his Tara. It looked good to me so I ordered one. As you can see below, it clips to the compression straps almost like it was made for it.
In his pictures on BCUSA, Creaky Bones has his waist pack mounted with its zipper towards the Tara, rather than away from it like I did. It might move less when done his way but you'd need to unclip it to access anything inside. It's easy enough to unclip it and reverse it if the situation dictates.
Of course, one could add a waist pack to the bottom of the Tara and possible use its belt, and then attach either another waist pack or sustainment pouch to the pack as shown above.
Obviously, at some point you are better off with a larger pack, but if the Tara is what you have you can extend its use with modular add-ons.