I must've missed something really basic, but how do you add more effects boxes in the signal path?

For example, if I have the Bender (Filter) pedal already in the path and want to add a Distortion box after it by dragging it into the path, then PODFarm forces me to replace the Bender pedal with the Distortion box.  I can understand boxes from the same category replacing each other, but these are from different categories (Filter and Distortion).  Thanks in advance.