I have bought a XOOM WiFI, and would like to know if it would be possible to connect it to internet using an external USB UMTS Stick (Huawei E122 HSPA USB Stick). I am aware this is not supported out of the box, I would like to know what would entail trying to connect it to a USB key which by default comes with drivers for desktop operating systems.
If you mean a USB ethernet adapter, then if you have one that has [fully open source] drivers in the mainline linux kernel, it may be as simple as recompiling the kernel for your xoom, installing it, and getting one of the USB otg adapter cables people have been using to read usb memory sticks on xooms. Fortunately the xoom seems more open to modified kernels than their phones.
Once it is available in the kernel, you may also need to figure out how to tell android to use that network interface - notice there's no ifconfig binary (though people have made their own). You might check with the android beagle board and android x86 projects to see how they initialize ethernet adapaters.