When fitting an outside tap, most are joined to the domestic cold water under the sink, where hopefully, there is enough room to work.
The connection does not have to be made under the sink but it is ideally made to a cold water pipe which is carrying cold water under mains pressure. To feed an outside tap from water coming from your cold water tank will seriously affect other peoples use of the water coming from this tank.
When mains water comes to a property it must feed at least one tap with mains pressure before it goes up to a cold water tank so double check that the pipe you want to connect to is under mains water pressure.