Just do:
? ? ? ?modprobe usb_serial vendor=0x0000 product=0x0000
with the proper vendor and product ids for your device, then plug it in.

No kernel changes needed at all, just have a pair of bulk in/out
endpoints and all will work automatically for you.

Thanks Greg, I had thought that the usbserial driver would only work
with proper CDC class virtual COM devices.
I compiled it and tried it on my device and it worked fine! I was able
to do bidirectional communication. So at least now I can be sure that
one end is working. But the same device with the same firmware still
doesn't work with the (customized) usb-skeleton. What could be wrong?

Regards,
Felix.