It 100% can
be added for sure, because CPUs in 750 and 750UP are the same.
If you trace up USB D+/D- tracks from the socket to the CPU, you will see that the only component you should add is L2 (and a USB socket, of course). L2 is a standard double ferrite coil, it is widely used in USB applications and can be easily bought. That's gonna be enough for self-powered devices like an external HDD with a dc adaptor. However, if you want to power up the device plugged in, you can go several ways:
1) Cheaper and easier one: if you only want to conect a standard flash device and you use power adaptor included with router,you can simply add a 7805 and a 470uF 10v cap after it, then connect 7805 input to dc source and output to +5v track on the pcb that leads to the usb socket. This will give you about 100-200 ma/5v source on the USB socket, and it's enough for most flash devices. And get 7805 insulated from main pcb for sure!
2) The right way: buy a small dc-dc 12v to 5v converter board, 500 ma output max, and wire it up to dc source and to the usb socket, place it somewhere inside the router box, not on the cpu preferrably
, no extra cap needed this time. And get it insulated from main pcb for sure too!
3) The hard way: get a photo of 750UP board, see what components are installed (or even better, get a such board) and solder these to your own board. This is not the best way, and it will require non-beginner soldering skills + some advanced soldering equipment (a soldering station, for example). And of course, you can easily damage your router components on the PCB.