Presumably, you were able to attach a USB external drive to the Home Hub previously when it was your main router?
If so, you need to disable the Hub's DHCP server so that, when it is connected to the Sky router, LAN port to Lan port, it will behave as a simple Network Switch or Wireless Access Point. Computers can then connect to it instead of the main router. This shouldn't really matter as the whole network will be in the same Subnet (IP address range).
You may need to use a "crossover" ethernet cable to connect the two routers.
>>> can I connect to two wireless signals, the HomeHub one and the SKY one for the internet?
This would only be possible if the PC had two Wireless Network Adapters installed.