how to measure mixer
it is difficult for u to simulate dc offset of the mixer in schematic design as all circuits and devices are matched. you could set mismatch on some devices, for example, the load resistors(the easiest case) and then simu the dc offset. dc offset is mainly caused by self-mixing, 2nd nonlinearity and mismatch, among which the last one could be simulated.
i have designed a direct-conversion mixer and i think it is unnecessary to simu dc offset of your mixer 'cause double-balanced mixer has little mismatch. you could set mismatch and then simu the ability to reduce dc offset