While doing more research on the Miix i found out that there are adapter modules around which offer an internal USB port within the mpcie port.
on the schematic drawing there are the pins described which should be Pin 36 and 38 and lead to a full USB2.0 support. I will investigate soon, if there are options at NGFF or M.2 availablebecause the mpcie port is missing in our miix.
so it could be possible that there is a way to get a signal transfern between an USB-Stick ( even if it is for storage or a functional device like 3G modem) and the mainboard. Lenovo hides technical informations about the used mainboards. It would help if there are datasheets around but for now it is not possible to find anything in the web.
Kris (one of our commenters) gave me a hint about the dell venue pro 8 where the tabletpcreview community get it managed to find out pins on the mainboard where it is possible to connect different usb devices to it LINK.
Due to the less different versions at the Dell their community is there much bigger. The mainboards in both devices the Dell Venue 8 and the Lenovo Miix 2 8 are totally different so it is not possible to sense out where the pins are at our devices.
—edit—
So like i thought, the NGFF or M.2 Standard provides USB2.0 support.
Now the things getting interesting…