I haven't used Cubase in a while, but if it's like the inserts in PT or DP, then IF you have an open space for another plug, then you should be able to grab the others and move down a spot without changing your adjustments. Then you should be able to add another into the open spot
In ProTools it was just a drag and drop, in Digital P, you have to wait for the cursor to change to a little hand (grabber), then drag and drop