How to simulate the parasitic passband of the low pass filters?

Hi, I am simulating a down-converter system in ADS. The schematic simulations allow to use the lowpass filters, without no parasitic passband feature, which is not the fact. How can I set the parasitic passband feature of the low pass filters in the ADS schematic, just use the if else equations? For example, a butterworth lowpass filter,of which the passband is 2GHz and the 40dB stopband is 3.5GHz, then to some extent, just for example, the parasitic passband appearing at 4.6GHz for about 20dB, how can I set the attributes in the low pass filter for butterworth? Thanks.