Can a resistor of known value be used to induce a predictable current, in accordance with Ohm's law?
If you have a 100A shunt it is already calibrated (probably within 0.25% or 0.5%). Just disconnect the shunt, apply 0mV to your readout circuit and make it read zero, then apply 75.00mV and make it read 100.0A (repeat if necessary). 

There is no need for a high current source unless you doubt the provenance of the shunt.