Im not sure if what your asking can be done as titles need to be installed through WUP installer manually (I think it can send over the network to your sd card) but also, if you want to install directly from wii u usb helper to your nand your taking a very big risk if the data gets corrupted.
Besides, the nand doesn't have a lot of space on it either (8gb or 32gb) so I'd recommend you bite the bullet and get a larger storage unit and sd. A 32gb sd card should suffice for most large games anyway and are dirt cheap these days.