On the time-frequency relationships of median filters
The median filter is actually an example of a non-linear filtering operation. 

This stands in contrast to linear filtering operations (the 'classical' way that involve the convolution of a filter's co-efficients with a signal). Since convolution in time is the same as multiplication in the frequency domain, you can go back and forth between the domains to figure out co-efficients of your filter in either basis. 

However, since medial filtering is a non-linear operation, you cannot do this. There is nothing you can convolve your signal with (ie, no linear operation you can do on your signal) such that you would get the same result as you see by using the non-linear medial filter.