An ip helper address converts broadcast traffic into unicast traffic.
For instance I often use it for DHCP to take DHCP broadcast from one broadcast domain and send it unicast to a DHCP on a different broadcast domain.
As far as I know you can only configure a helper address under an interface as it needs to know where to source the unicast traffic from.
If you need to do it on all your interfaces you would just apply it to each interface.
Can you go into more detail about why you would need to apply it globally or what the device you are trying to apply it on is?
