So, when I bought my 2012 Mazda3s Touring, the sales person (or lack there of) said there was one in the glovebox, and he was liar. There isn't a way to add USB to the system unless you replace the system, which because Bose, isn't a fun task (nor a cheap one).
One thing you could do is get an old school iPod Classic (or large capacity MP3 player), and use that Aux IN in the center console, but the sound may be a little stank in comparison to using USB or BT.
I guess add more memory to your Smartphone and use BT would be th best option. Hope this info helps!
For my 2012 GX (Canadian model) there was a Ipod specific adapter as an OEM accessory that allowed you to connect your ipod via usb and control it with your head unit. However this was at least $300.00 for the kit. you can try searching ebay (but they were rare back in 2012).
The aux in in the center console and streaming via bluetooth have both been great for me.