Low amplitude curent measurement
Yes, you can buy a multimeter that can measure 6nA. For example, the 7.5 digit 34470A has a 1uA full-scale DC current range, accuracy in the 50pA neighborhood. It's a nice piece of kit, but a bit expensive.
For cheap measurement you can use a common handheld multimeter if you know the input resistance. For example, the +/-199.9mV range on a 3.5 digit multimeter with 10M\$\Omega\$ input resistance acts as a DC ammeter with a full scale range of 19.99nA.
If you want to make a circuit, a useful part (perhaps gross overkill) is the LMC6001, which has a guaranteed input bias current at room temperature of less than 25fA (10fA typical), which is ~0.0004% (~0.0002% typical) of the current you want to measure. That and a resistor will do it. Of course you need to take extreme measures to get full performance out of that op-amp, but 6nA is actually a pretty heavy current. Avoid conductive fluxes and keep things very clean.