Can I set the test current when making resistance measurements with the Keysight 34401A?

No, the test current is set depending on the range of the instrument. The specifications are the following:Range Test Current100.0000 ohm 1 mA1.000000 kohm 1 mA10.00000 kohm 100 uA100.0000 kohm 10 uA1.000000 Mohm 5 uA10.00000 Mohm 500 nA100.0000 Mohm 500nA||10MohmIf you need a fixed source current, we suggest using an external current source. You may then measure the voltage across the device under test and use Ohm's Law to find the resistance.